Computational
Organo-
Metallic and
Inorganic
Chemistry
Organo-
Metallic and
Inorganic
Chemistry
COMIC group
Two hitherto unknown organometallic compounds with antitumor activity, [Ru(η6-2-(1-propenyl)anisole)(en)(Cl)]PF6 (3) and [Ru(η6-2-(1-propenyl)anisole)(en)(l)]PF6 (4), where en is ethylenediamine, were synthesized and completely characterized using standard techniques (1H and 13C NMR, high-resolution MS and elemental analysis). The lipophilicity and hydrolysis rate kinetics were assessed and compared to the previously reported [Ru(η6-4-(1-propenyl)anisole)(en)(halogen)]PF6 derivatives (4-(1-propenyl)anisole or anethole), where the halogen is Cl (1) or I (2). Based on the obtained rate constants, the coordination of (1-propenyl)anisole to the Ru(en) moiety yielded organometallic compounds that are as active as compounds that bind p-cymene as the arene ligand. Consistent with previously reported kinetic data, our density functional theory-based computational study revealed that an associative interchange mechanism predominates in the hydrolysis of this type of compound, and only small variations (∼1 kcal mol−1) were observed between the stabilities of the transition states corresponding to different derivatives. In vitro analyses of the anti-proliferative activity revealed that compounds 1 to 3 generally exhibit better cytotoxicity and selectivity (tumor versus non tumor cells) toward the gastric tumor cell lines AGS and SNU-1, compared to the parent [Ru(η6-p-cymene)(en)X]PF6 (X: Cl and I) systems. Compound 3 showed similar cytotoxicity to compound 1 toward the AGS cell line, indicating that the change in the substitution pattern of the coordinated arene from 4-(1-propenyl)anisole to 2-(1-propenyl)anisole did not prominently affect the biological behavior. Compound 2 remained the most promising candidate to treat gastric cancer.
Transmetallation studies with the phosphaethynolate ion, [OCP]−, have largely resulted in coordination according to classical Lewis acid–base theory. That is, for harder early transition metal ions, O-bound coordination has been observed, whereas in the case of softer late transition metal ions, P-bound coordination predominates. Herein, we report the use of a V(III) complex, namely [(nacnac)VCl(OAr)] (1) (nacnac− = [ArNC(CH3)]2CH; Ar = 2,6-iPr2C6H3), to transmetallate [OCP]− and bind via the P-atom as [(nacnac)V(OAr)(PCO)] (2), the first example of a 3d early transition metal that binds [OCP]−via the P-atom. Full characterization studies of this molecule including HFEPR spectroscopy, SQuID measurements, and theoretical studies are presented.
The seminal contributions by Sonogashira, Cassar and Heck in mid 1970s on Pd/Cu- and Pd-catalysed (copper-free) coupling of acetylenes with aryl or vinyl halides have evolved in myriad applications. Despite the enormous success both in academia and in industry, however, critical mechanistic questions of this cross-coupling process remain unresolved. In this study, experimental evidence and computational support is provided for the mechanism of copper-free Sonogashira cross-coupling reaction. In contrast to the consensus monometallic mechanism, the revealed pathway proceeds through a tandem Pd/Pd cycle linked via a multistep transmetallation process. This cycle is virtually identical to the Pd/Cu tandem mechanism of copper co-catalysed Sonogashira cross-couplings, but the role of CuI is played by a set of PdII species. Phosphine dissociation from the square-planar reactants to form transient three-coordinate Pd species initiates transmetallation and represents the rate-determining step of the process.
In this work, we prepared, isolated, and structurally characterized a zirconium complex having a terminally bound imide motif, (PN)2Zr≡NH (PN– = (N-(2-iPr2P-4-methylphenyl)-2,4,6-trimethylanilide)), along with the zirconium nitride complex {(PN)2Zr≡N[μ2-Li(THF)]}2. (PN)2Zr≡NH was prepared by reduction of trans-(PN)2Zr(N3)2 with KC8. Isotopic labeling and spectroscopic studies were conducted using the respective 15N enriched isotopologues, whereas solid-state structural studies confirmed some of the shortest Zr≡N distances known to date (Zr≡NH, 1.830(3) Å; Zr≡N–, 1.822(2) Å). It was found that the nitride in {(PN)2Zr≡N[μ2-Li(THF)]}2 is super basic and in the range of −36 to −43 pKb units. Computational studies have been applied to probe the bonding and structure for this new class of zirconium–nitrogen multiple bonds.
The particle-particle random phase approximation (pp-RPA) has been deployed to study the spin-state energetics of transition metal (TM) complexes for the first time in this work. Namely, we designed and implemented a non-canonical reference pp-RPA protocol that is capable of capturing the singlet low-spin (LS) – triplet intermediate-spin (IS) excitation process of iron(II) complexes; herein we applied this method to iron-porphyrin related heme derivatives with clearly defined LS and IS electronic states. Coupled to the CAM-B3LYP functional and to Dunning-type basis sets, we utilized both the active-space and Davidson methods to solve the pp-RPA equation effectively to obtain vertical singlet–triplet excitation energies. Correcting these vertical metrics with a structural relaxation factor for each species, we evaluated the relative stability of LS and IS electronic states. Comparison of the pp-RPA results to established ab initio data revealed that pp-RPA describes well excitation energies and related relative spin state stabilities if the transition is based on non-bonding d-orbitals, such as complexes without an axial ligand in the investigated set of molecules. But it notably overestimates the stability of the singlet LS state to the triplet IS state in complexes, where the d-orbitals at which the excitation is centered have bonding or antibonding character.
The mononuclear niobium methylidyne [(PNP)(ArO)Nb≡CH] (1; PNP– = N[2-PiPr2-4-methylphenyl]2–, Ar = 2,6-iPr2C6H3) reacts with the isocyanate O═C═NtBu to form a mononuclear niobium oxo species with a rare example of an azaallenyl ligand, namely [(PNP)(ArO)Nb═O(CH═C═NtBu)] (2). When 1 is treated with the phosphaalkyne P≡CAd (Ad = 1-adamantyl), P≡C bond cleavage occurs to form a mononuclear complex where P–P coupling has occurred between the formal phosphaalkyne phosphorus atom and one phosphine arm from the PNP ligand, namely [(PNPP)(ArO)Nb(η2-AdCCH)] (3). Solid-state structural studies and isotopic labeling experiments confirm C–C bond formation of the methylidyne group as well as provide conclusive evidence for the oxo ligand in 2 being terminal and the fate of the phosphorus atom from P≡CAd in complex 3. Computational studies have been applied to understand the pathway involving the P–P bond forming reaction of 1 and P≡CAd.
The first example of the OCPPCO ligand, diisophosphaethynolate, is reported via reductive coupling of an Sc-OCP precursor. Upon reduction with KC8, isolation of the dinuclear complex, namely [K(OEt2)]2[(nacnac)Sc(OAr)]2(OCPPCO), is observed, leading to a unique motif [OCPPCO]4-, stabilized by two scandium centers. We report detailed NMR spectra of all complexes as well as IR and single crystal X-ray studies to fully elucidate the nature of these complexes in solution as well the solid state. Theory is combined to probe the electronic structure and orbitals responsible for the bonding interactions in Sc-OCPPCO-Sc skeleton but also compared to the linear mode observed in the precursor.
In this study we aimed at deriving an intuitive concept that explains the characteristic dependence of spin-state energetics on the exact exchange admixture of DFT functionals in the case of octahedral transition metal complexes. We analyzed the change of electron density distributions upon varying the admixture, c3, in B3LYP for archetype ionic and covalent systems as well as for Fe2+ ion in an ideal octahedral field. We also attempted to understand how the DFT description of the electronic structure of octahedral complexes changes as a function of c3. We also present a systematic spin-state energy analysis of 50 octahedral complexes of various metals and ligands with consistent experimental data allowing us to derive, in theory, an optimal c3 value for each system. We put forward the notion that the admixture dependence of DFT spin-state energetics stems from the treatment of nondynamic correlation arising from the mixing of (M-Lz2)0(dz2)2 and (M-Lx2-y2)0(dx2-y2)2 configurations into the dominant (M-Lx2-y2)2(dx2-y2)0 and (M-Lx2-y2)2(dx2-y2)0 ones in the low(er) spin states. Namely, in the effort to mimic such electron-electron interactions, ExLDA overestimates while exact exchange down plays the contribution of this type of electron correlation to the stability of low(er) spin states leading to the widespread practical observation that the higher the exact exchange admixture the more stable the high-spin state configuration is.
The chalcogen bond has been acknowledged as an influential noncovalent interaction (NCI) between an electron-deficient chalcogen (donor) and a Lewis base (acceptor). This work explores the main features of chalcogen bonding through a large-scale computational study on a series of donors and acceptors spanning a wide range in strength and character of this type of bond: (benzo)chalcogenadiazoles (with Ch = Te/Se/S) versus halides and neutral Lewis bases with O, N, and C as donor atoms. We start from Pearson's hard and soft acids and bases (HSAB) principle, where the hard nature of the chalcogen bond is quantified through the molecular electrostatic potential and the soft nature through the Fukui function. The σ-holes are more pronounced when going down in the periodic table and their directionality matches the structural orientation of donors and acceptors in the complexes. The Fukui functions point toward an n→σ*-type interaction. The initial conjectures are further scrutinized using quantum mechanical methods, mostly relating to the systems' electron density. A Ziegler–Rauk energy decomposition analysis shows that electrostatics plays a distinctly larger role for the soft halides than for the hard, uncharged acceptors, associated with the softness matching within the HSAB principle. The natural orbital for chemical valence analysis confirms the n→σ* electron donation mechanism. Finally, the electron density and local density energy at the bond critical point in the quantum theory of atoms in molecules study and the position of the spikes in the reduced density gradient versus density plot in the NCI theory situate the chalcogen bond in the same range as strong hydrogen bonds.
Using a set of state-of-the-art quantum chemical techniques we scrutinized the characteristically different reactivity of frustrated and classical Lewis pairs towards molecular hydrogen. The mechanisms and reaction profiles computed for the H2 splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H2 activation. More importantly, the analysis of activation parameters unambiguously revealed the existence of two reaction pathways through a low-energy and a high-energy transition state. An exhaustive scrutiny of these transition states, including their stability, geometry and electronic structure, reflects that the electronic rearrangement in low-energy transition states is fundamentally different from that of high-energy transition states. Our findings reveal that the widespread consensus mechanism of H2 splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H2-activating systems. One of the criteria of H2-activation, actually, is the availability of a low-energy transition state that represents a different H2 splitting mechanism, in which the electrostatic field generated in the cavity of Lewis pair plays a critical role: to induce a strong polarization of H2 that facilities an efficient end-on acid-H2 interaction and to stabilize the charge separated “H+–H−” moiety in the transition state.
Utilizing the bulky guanidinate ligand [LAr*]- (LAr* = (Ar*N)2C(R), Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl, R = NCtBu2) for kinetic stabilization, the synthesis of a rare terminal Fe(IV) nitride complex is reported. UV irradiation of a pyridine solution of the Fe(II) azide [LAr*]Fe(N3)(py) (3-py) at 0 °C cleanly generates the Fe(IV) nitride [LAr*]FeN(py) (1). The 15N NMR spectrum of the 115N (50% Fe≡15N) isotopomer shows a resonance at 1016 ppm (vs externally referenced CH3NO2 at 380 ppm), comparable to that known for other terminal iron nitrides. Notably, the computed structure of 1 reveals an iron center with distorted tetrahedral geometry, τ4 = 0.72, featuring a short Fe≡N bond (1.52 Å). Inspection of the frontier orbital ordering of 1 shows a relatively small HOMO/LUMO gap with the LUMO comprised by Fe(dxz,yz)-N(px,y) π*-orbitals, a splitting that is manifested in the electronic absorption spectrum of 1 (λ = 610 nm, ε = 1375 L·mol-1·cm-1). Complex 1 persists in low-temperature solutions of pyridine but becomes unstable at room temperature, gradually converting to the Fe(II) hydrazide product [κ2-(tBu2CN)C(η6-NAr*)(N-NAr*)]Fe (4) upon standing via intramolecular N-atom insertion. This reactivity of the Fe≡N functionality was assessed through molecular orbital analysis, which suggests electrophilic character at the nitride functionality. Accordingly, treatment of 1 with the nucleophiles PMe2Ph and Ar-N≡C (Ar = 2,6-dimethylphenyl) lead to partial N-atom transfer and formation of the Fe(II) addition products [LAr*]Fe(N=PMe2Ph)(py) (5) and [LAr*]Fe(N=C=NAr)(py) (6). Similarly, 1 reacts with PhSiH3 to give [LAr*]Fe[N(H)(SiH2Ph)](py) (7) which Fukui analysis shows to proceed via electrophilic insertion of the nitride into the Si–H bond.
The first structural elucidation of the first group 4 transition metal P2 complex, namely [(nacnac)Ti(OAr)]2(μ2:η2,η2-P2) (1), is reported. Complex 1 is formed via reductive decarbonylation of the phosphaethynolate ion -[OCP], which serves as a P atom source. The rhombic Ti2P2 core is essentially planar with short bond lengths suggesting some degree of multiple bonding character between the Ti-P and P-P sites. Also observed in 1 was the rather downfield shifted 31P NMR spectroscopic signal, appearing as a broadened resonance (Δν1/2 ~ 4034 Hz) at 907 ppm. Computational studies of 1 provide an understanding of the Ti2P2 core as well as the origin of the highly downfield 31P NMR spectroscopic signal.
In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ2-K(OEt2)]2[(PN)2Ti[triple bond, length as m-dash]N]2 (1) (PN− = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pKa range of the NH group in (PN)2Ti[triple bond, length as m-dash]NH to be between 26–36. Complex 1 could be produced by a reductively promoted elimination of N2 from the azide precursor (PN)2TiN3, whereas reductive splitting of N2 could not be achieved using the complex (PN)2Ti[double bond, length as m-dash]N[double bond, length as m-dash]N[double bond, length as m-dash]Ti(PN)2 (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O)tBu and OCCPh2 to form NCtBu and KNCCPh2, respectively, along with the terminal oxo complex (PN)2Ti[triple bond, length as m-dash]O, which was also characterized. A combination of solid state 15N NMR (MAS) and 
theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN)2Ti[triple bond, length as m-dash]N], and the discrete salt [K(2,2,2-Kryptofix)][(PN)2Ti[triple bond, length as m-dash]N] as well as the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of 15Nnitride resonance in the 15N NMR spectrum was found to be linked to the K+ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.
The most relevant manifestations of ligand noninnocence of quinone and bipyridine derivatives are thoroughly scrutinized and discussed through an extensive and systematic set of octahedral ruthenium complexes, [(en)2RuL]z, in four oxidation states (z = +3, +2, +1, and 0). The characteristic structural deformation of ligands upon coordination/noninnocence is put into context with the underlying electronic structure of the complexes and its change upon reduction. In addition, by means of decomposing the corresponding reductions into electron transfer and structural relaxation subprocesses, the energetic contribution of these structural deformations to the redox energetics is revealed. The change of molecular electron density upon metal- and ligand-centered reductions is also visualized and shown to provide novel insights into the corresponding redox processes. Moreover, the charge distribution of the π-subspace is straightforwardly examined and used as indicator of ligand noninnocence in the distinct oxidation states of the complexes. The aromatization/dearomatization processes of ligand backbones are also monitored using magnetic (NICS) and electronic (PDI) indicators of aromaticity, 
and the consequences to noninnocent behavior are discussed. Finally, the recently developed effective oxidation state (EOS) analysis is utilized, on the one hand, to test its applicability for complexes containing noninnocent ligands, and, on the other hand, to provide new insights into the magnitude of state mixings in the investigated complexes. The effect of ligand substitution, nature of donor atom, ligand frame modification on these manifestations, and measures is discussed in an intuitive and pedagogical manner.
The role of the solvent and the influence of dynamics on the kinetics and mechanism of the SNAr reaction of several halonitrobenzenes in liquid ammonia, using both static calculations and dynamic ab initio molecular dynamics simulations, are investigated. A combination of metadynamics and committor analysis methods reveals how this reaction can change from a concerted, one-step mechanism in gas phase to a stepwise pathway, involving a metastable Meisenheimer complex, in liquid ammonia. This clearly establishes, among others, the important role of the solvent and highlights the fact that accurately treating 
solvation is of crucial importance to correctly unravel the reaction mechanism. It is indeed shown that H-bond formation of the reacting NH3 with the solvent drastically reduces the barrier of NH3 addition. The halide elimination step, however, is greatly facilitated by proton transfer from the reacting NH3 to the solvent. Furthermore, the free energy surface strongly depends on the halide substituent and the number of electron-withdrawing nitro substituents.
The metal-free reaction of terminal arylacetylenes with α,α-dichloroaldimines in 1,1,1,3,3,3-hexafluoro-2-propanol as the sole solvent results in the rapid and selective formation of γ,γ-dichloro-β-amino ketones. In this solvent the expected dichlorinated propargylamines and/or allylic amines are not formed. The dichloromethylene moiety of the aldimine acts as an activating group and is essential to accomplish this transformation. Electron-rich acetylenes lead to the best results and work well with all imines (with or without α′-H at the nitrogen substituent), while electron-deficient acetylenes only reacted with N-tert-butylaldimines (no α′-H). The mechanistic pathway showed 1,1,1,3,3,3-hexafluoro-2-propanol to protonate the aldimine, 
which in the rate-determining step will react with the arylacetylene to form a resonance-stabilized allene cation, which is trapped by a HFIP molecule giving rise to an enol ether, which promptly hydrolyzes to furnish exclusively the β-amino ketones. Using DFT techniques we found that the first C[BOND]C bond forming step is the rate-determining step and is associated with a barrier of about 21 kcal mol−1.
The synthesis and characterization of two high-valent vanadium–cyclo-P3 complexes, (nacnac)V(cyclo-P3)(Ntolyl2) (1) and (nacnac)V(cyclo-P3)(OAr) (2), and an inverted sandwich derivative, [(nacnac)V(Ntolyl2)]2(μ2-η3:η2-cyclo-P3) (3), are presented. These novel complexes are prepared by activating white phosphorus (P4) with three-coordinate vanadium(II) precursors. Structural metrics, redox behavior, and DFT electronic structure analysis indicate that a [cyclo-P3]3– ligand is bound to a V(V) center in monomeric species 1 and 2. A salient feature of these new cyclo-P3 complexes is their significantly downfield shifted (by ∼300 ppm) 31P NMR resonances, which is highly unusual compared to related complexes such as (Ar[iPr]N)3Mo(cyclo-P3) (4) and other cyclo-P3 complexes that display significantly upfield shifted resonances. 
This NMR spectroscopic signature was thus far thought to be a diagnostic property for the cyclo-P3 ligand related to its acute endocyclic angle. Using DFT calculations, we scrutinized and conceptualized the origin of the unusual chemical shifts seen in this new class of complexes. Our analysis provides an intuitive rational paradigm for understanding the experimental 31P NMR spectroscopic signature by relating the nuclear magnetic shielding with the electronic structure of the molecule, especially with the characteristics of metal–cyclo-P3 bonding.
The Ti(III) azido complex (PN)2Ti(N3) (PN– = (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide), can be reduced with KC8 to afford the nitride salt [μ2-K(OEt2)]2[(PN)2Ti≡N]2 in excellent yield. 
While treatment of the dimer with 18-crown-6 yields a mononuclear nitride, complete encapsulation of the alkali metal with cryptand provides the terminally bound nitride as a discrete salt [K(2,2,2-Kryptofix)][(PN)2Ti≡N]. All complexes reported here have been structurally confirmed and also spectroscopically, and the Ti–Nnitride bonding has been probed theoretically via DFT-based methods.
The heretofore unpredictable behavior of [26] and [28]hexaphyrins upon metalation has been elucidated through quantum chemical calculations. 
It is demonstrated that the molecular topology of Group 10 and Group 11 metal complexes of hexaphyrins depends on sensitive interplay between the intrinsic ligand strain and the metal–ligand interaction strength. As such, the aromaticity of the ligand and effective charge of the metal are revealed as key factors determining the binding mode and the preference for Möbius or Hückel structures. These findings offer a new perspective to rationalize experimental observations for metalated hexaphyrins. More importantly, the proposed guidelines could be useful for designing novel complexes of hexaphyrins, such as a hitherto unknown Möbius [26]hexaphyrin complex.
The heretofore unpredictable behavior of [26] and [28]hexaphyrins upon metalation has been elucidated through quantum chemical calculations. 
It is demonstrated that the molecular topology of Group 10 and Group 11 metal complexes of hexaphyrins depends on sensitive interplay between the intrinsic ligand strain and the metal–ligand interaction strength. As such, the aromaticity of the ligand and effective charge of the metal are revealed as key factors determining the binding mode and the preference for Möbius or Hückel structures. These findings offer a new perspective to rationalize experimental observations for metalated hexaphyrins. More importantly, the proposed guidelines could be useful for designing novel complexes of hexaphyrins, such as a hitherto unknown Möbius [26]hexaphyrin complex.
Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal–ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type ligands. 
Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or suppress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow rational redox-leveling, i.e. the moderation of redox potentials of ligand-centred electron transfer processes, potentially leading to catalysts with low overpotential in multielectron activation processes.
The contributions of covalent and noncovalent interactions to the formation of classical adducts of bulky Lewis acids and bases and frustrated Lewis pairs (FLPs) were scrutinized by using various conceptual quantum chemical techniques. Significantly negative complexation energies were calculated for fourteen investigated Lewis pairs containing bases and acids with substituents of various sizes. A Ziegler–Rauk-type energy decomposition analysis confirmed that two types of Lewis pairs can be distinguished on the basis of the nature of the primary interactions between reactants; dative-bond formation and concomitant charge transfer from the Lewis base to the acid is the dominant and most stabilizing factor in the formation of Lewis acid–base adducts, whereas weak interactions are the main thermodynamic driving force (>50 %) for FLPs. Moreover, the ease and extent of structural deformation of the monomers appears to be a key component 
in the formation of the former type of Lewis pairs. A Natural Orbital for Chemical Valence (NOCV) analysis, which was used to visualize and quantify the charge transfer between the base and the acid, clearly showed the importance and lack of this type of interaction for adducts and FLPs, respectively. The Noncovalent Interaction (NCI) method revealed several kinds of weak interactions between the acid and base components, such as dispersion, π–π stacking, C[BOND]H⋅⋅⋅π interaction, weak hydrogen bonding, halogen bonding, and weak acid–base interactions, whereas the Quantum Theory of Atoms in Molecules (QTAIM) provided further conceptual insight into strong acid–base interactions.
Inspection of the electrostatic potential of C2F3X (X = F, Cl, Br, and I) revealed a second electropositive region in the immediate vicinity of the C═C double bond apart from the σ hole of chlorine, bromine, and iodine, leading to C(sp2)–X···Y halogen bonding, through which complexes stabilized by so-called lone pair···π interactions can be formed. Consequently, the experimental studies for the complexes of dimethyl ether with C2F3X (X = F, Cl, Br, and I) not only allowed one to experimentally characterize and rationalize the effects of hybridization on halogen bonding but, for the first time, also allowed the competition of C–X···Y halogen bonding and lone pair···π interactions to be studied at thermodynamic equilibrium. Analysis of the infrared and Raman spectra reveals that in the cryosolutions of dimethyl ether and C2F3I, solely the halogen-bonded complex is present, whereas C2F3Br and C2F3Cl give rise to a lone pair···π bonded complex as well as a halogen-bonded complex. Mixtures of dimethyl 
ether with C2F4 solely yield a lone pair···π bonded complex. The experimentally derived complexation enthalpies for the halogen bonded complexes are found to be −14.2(5) kJ mol–1 for C2F3I·DME and −9.3(5) kJ mol–1 for C2F3Br·DME. For the complexes of C2F3Cl with dimethyl ether, no experimental complexation enthalpy could be obtained, whereas the C2F4·DME complex has a complexation enthalpy of −5.5(3) kJ mol–1. The observed trends have been rationalized with the aid of an interaction energy decomposition analysis (EDA) coupled to a Natural Orbital for Chemical Valence (NOCV) analysis and also using the noncovalent interaction index method.
The nature and origin of ion-π and ion-σ interactions has been systematically investigated using dispersion-corrected density functional theory and the recently developed noncovalent interaction (NCI) method. A detailed analysis of these interactions is performed with the aim to identify the requirements that have to be fulfilled by the molecular system for strong ion-ligand interactions. Interestingly, our results indicate that aliphatic systems, such as cyclohexane, can interact as strong as aromatic ones with both cations and anions, despite of having a negligible quadrupole moment. In fact, cyclohexane binds anions stronger than benzene itself but slightly weaker that hexafluorobenzene. The NCI method reveals that the interaction between the ions and three C–H bonds of the saturated fragment are responsible for the surprisingly strong ion-σ interaction. A weakening of the ion-σ interactions is observed in the order: Li+ > F− > Na+ > Cl− > Br− ≈ K+. In addition, a complete Ziegler–Rauk type energy decomposition analysis has been carried out in order to reveal the origins of the thermodynamic driving force for complex formations. 
The electron density deformation upon complex formation has been scrutinized with a complementary NOCV analysis allowing the identification of molecular orbital interaction contributions to the stabilization. Based on these analysis, it is shown that the formally anion-π interaction is rather an anion-σ∗ interaction.
Treatment of [TaCl2(CH3)3] with 2 equiv of NaOAr′ (OAr′ = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide) yields cleanly the bis-aryloxide trimethyl complex [(Ar′O)2Ta(CH3)3] (1), which is isolated in 92% yield and is spectroscopically and structurally characterized. Addition of 2 equiv of HOAr′ to [TaCl2(CH3)3] results in clean protonation concurrent with formation of the bis-aryloxide methyl derivative [(Ar′O)2Ta(CH3)Cl2] (2), which was also fully characterized, including an X-ray structure. Despite being close derivatives, complex 1 (trigonal bipyramidal) and 2 (square pyramidal) possess very different structures, with the e set in a square-pyramidal molecular orbital diagram being
key to their preferred geometry. Addition of excess ylide, H2CPPh3, to 2 results in formation of the terminal tantalum methylidene chloride complex [(Ar′O)2Ta═CH2(Cl)(H2CPPh3)] (3) in 64% yield, which is characterized by multinuclear NMR spectroscopy and a solid-state structure determination.
Homobimetallic metallophilic interactions between copper, silver, and gold-based [(NHC)MX]-type complexes (NHC=N-heterocyclic carbene, i.e, 1,3,4-trimethyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene; X=F, Cl, Br, I) were investigated by means of ab initio interaction energies, Ziegler–Rauk-type energy-decomposition analysis, the natural orbital for chemical valence (NOCV) framework, and the noncovalent interaction (NCI) index. It was found that the dimers of these complexes predominantly adopt a head-to-tail arrangement with typical M⋅⋅⋅M distance of 3.04–3.64 Å, in good agreement with the experimental X-ray structure determined for [{(NHC)AuCl}2], which has an Au⋅⋅⋅Au distance of 3.33 Å. The interaction energies between silver- and gold-based monomers are calculated to be about −25 kcal mol−1, whereas that for the Cu congener is significantly lower (−19.7 kcal mol−1). With the inclusion of thermal and solvent contributions, both of which are destabilizing, by about 15 and 8 kcal mol−1, respectively, an equilibrium process is predicted for the formation of dimer complexes. Energy-decomposition analysis revealed a dominant electrostatic contribution to the interaction energy, besides significantly stabilizing dispersion and orbital interactions. 
This electrostatic contribution is rationalized by NHC(δ+)⋅⋅⋅halogen(δ−) interactions between monomers, as demonstrated by electrostatic potentials and derived charges. The dominant NOCV orbital indicates weakening of the π backdonation in the monomers on dimer formation, whereas the second most dominant NOCV represents an electron-density deformation according to the formation of a very weak M⋅⋅⋅M bond. One of the characteristic signals found in the reduced density gradient versus electron density diagram corresponds to the noncovalent interactions between the metal centers of the monomers in the NCI plots, which is the manifestation of metallophilic interaction.
Insight into the key factors driving the competition of halogen and hydrogen bonds is obtained by studying the affinity of the Lewis bases trimethylamine (TMA), dimethyl ether (DME), and methyl fluoride (MF) towards difluoroiodomethane (CHF2I). Analysis of the infrared and Raman spectra of solutions in liquid krypton containing mixtures of TMA and CHF2I and of DME and CHF2I reveals that for these Lewis bases hydrogen and halogen-bonded complexes appear simultaneously. In contrast, 
only a hydrogen-bonded complex is formed for the mixtures of CHF2I and MF. The complexation enthalpies for the C[BOND]H⋅⋅⋅Y hydrogen-bonded complexes with TMA, DME, and MF are determined to be −14.7(2), −10.5(5) and −5.1(6) kJ mol−1, respectively. The values for the C[BOND]I⋅⋅⋅Y halogen-bonded isomers are −19.0(3) kJ mol−1 for TMA and −9.9(8) kJ mol−1 for DME. Generalization of the observed trends suggests that, at least for the bases studied here, softer Lewis bases such as TMA favor halogen bonding, whereas harder bases such as MF show a substantial preference for hydrogen bonding.
Novel spiropseudoindoxyls were synthesized in high yields via a fully regioselective Au(III)-catalyzed cycloisomerization of easily obtainable o-nitrophenylpropiolamides, followed by an intramolecular dipolar cycloaddition as key steps. 
This one-pot cascade reaction resulted in new tetracyclic pseudoindoxyls, which were hydrogenated toward the title compounds as single diastereomers via N–O cleavage. The mechanism of the gold catalyzed cycloisomerization was studied by DFT and suggests a reaction path without the intermediacy of gold carbenoid species in these cases.
The dimethyl aryloxide complexes [(PNP)M(CH3)2(OAr)] (M=Zr or Hf; PNP−=N[2-P(CHMe2)2-4-methylphenyl]2); Ar=2,6-iPr2C6H3), which were readily prepared from [(PNP)M(CH3)3] by alcoholysis with HOAr, undergo photolytically induced α-hydrogen abstraction to cleanly produce complexes [(PNP)M=CH2(OAr)] with terminal methylidene ligands. These unique systems have been fully characterized, including the determination of a solid-state structure in the case of M=Zr.
The alkyne complexes [(PNP)Ti(η2-HC≡CH)(CH2tBu)] (2) and [(PNP)Ti(η2-HC≡CMe)(CH2tBu)] (3) have been prepared by treatment of [(PNP)Ti═CHtBu(OTf)] (1) with the Grignard reagents H2C═CHMgCl and MeHC═CHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2C═C(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(η2-MeC≡CMe)(CH2tBu)] (4) can be similarly prepared from 1 and MeHC═C(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (1H, 13C, and 31P) NMR spectroscopic experiments, including selectively 31P decoupled 1H{31P}, 1H–31P HMBC, 1H–31P HOESY, and 31P EXSY. Variable-temperature 1H and 31P{1H} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol–1, and such a process has been corroborated by theoretical studies. 
Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)Ti═CHtBu(CH═CHR)] followed by a concerted, metal-mediated β-hydrogen abstraction step that has been computed to have a barrier of 20–22 kcal mol–1. The geometry and rotational mechanism of the alkyne ligand in 2 are presented and compared with those of the ethylene derivative [(PNP)Ti(η2-H2C═CH2)(CH2tBu)] (5), which does not display rotation of the bound ethylene under the same conditions.
The trans effect and trans influence were investigated and rationalized in the aminolysis, a typical nucleophilic substitution reaction, of trans-TPtCl2NH3 complexes (T = NH3, PH3, CO and C2H4) using energy decomposition analysis, both along the reaction paths and on the stationary points, and Natural Orbital for Chemical Valence analysis. In order to scrutinize the underlying principles and the origin of the kinetic trans effect, plausible structural constraints were introduced in the decomposition analysis, which allowed eliminating the distance dependence of the interaction energy components. It was established that the trans effect can be rationalized with the interaction of the TPtCl2 and NH3 fragments in the reactant state and TPtCl2 and (NH3)2 fragments in the transition state. It was evinced quantitatively that the [sigma]-donor ability of T indeed controls the stability of the reactant, whereas in the case of [small pi]-acids, backdonation stabilizes the transition state, for which conceptually two mechanisms are available: intrinsic and induced [small pi]-backdonation. In the destabilization of the reactant and also in the labilization of the leaving group (trans influence) repulsion plays a more important role than orbital sharing effects, which are the cornerstones of the widely accepted interpretations
of the trans influence, such as competition for donation or limitation of the donation of the leaving group by the trans ligand T. This repulsive interaction was rationalized both in terms of donated electron density and also in the molecular orbital framework. NOCV orbitals indeed clearly show that the [sigma]-trans effect can be envisioned as a donation from the trans ligand not only to the metal but also to the [sigma]* orbital of the metal-leaving group bond, which manifests as a repulsion between the metal and the leaving group.
2-Picolyl azide reacts with cis-[PtCl2(DMSO)2] to form the diimino complex [PtIICl2{NH═C(H)Py}] with subsequent dinitrogen 
liberation. The formation of the latter complex is scrutinized in a combined experimental and theoretical analysis. We establish in silico that the transformation involves a highly reactive intermediate containing a Pt═N double bond formed after the extrusion of N2 from the azide functionality. The prerequisites for N2 liberation and for the stabilization of the nitrene-related intermediate are analyzed in detail.
The intrinsic electrophilic reactivity of linear acenes up to octacene was investigated by using interaction energy potentials and their components, namely Pauli repulsion, orbital interaction, and electrostatic interaction potentials using the Ziegler–Rauk decomposition scheme. We found that the shared regions above C1 and C2 of the outer ring are slightly more attractive towards an incoming electrophile than the inner ring secondary carbon atoms, mostly due to a more advantageous electrostatic interaction. This result agrees with the stability order of prereactive π complexes formed between anthracene and the electrophilic HCl. Decomposition potentials determined in the bond formation regime indicate that longer acenes become more reactive because of the increasing orbital interaction. In addition, the orbital interaction potentials, which represent the overall effect of the high lying reactive orbital, exhibit straightforward patterns, predicting strongly preferred inner ring secondary carbon atoms over outer ring carbon atoms. Such an orbital-interaction controlled regioselectivity shows up in the total interaction energy potentials 
when the electrostatic interaction is scaled down, predicting the functionalization of acenes with electrophiles on the inner secondary carbon atoms, in agreement with the experimental regioselectivity. Upon analyzing the stability of transition states for the addition of HCl to anthracene, the Pauli repulsion shows up to be an important factor in controlling the stability of the transition states and favoring functionalization along the central C9 and C10 positions to outer ring carbon atoms. The resonance stabilization concept, which is generally accepted to account for the regioselectivity of higher acenes, could not be evidenced by our analysis.
Halogen bonds between the trifluoromethyl halides CF3Cl, CF3Br and CF3I, and dimethyl ether, dimethyl sulfide, trimethylamine and trimethyl phosphine were investigated using Pearson’s hard and soft acids and bases (HSAB) concept with conceptual DFT reactivity indices, the Ziegler–Rauk-type energy-decomposition analysis, the natural orbital for chemical valence (NOCV) framework and the non-covalent interaction (NCI) index. It is found that the relative importance of electrostatic and orbital (charge transfer) interactions varies as a function of both the donor and acceptor molecules. Hard and soft interactions were distinguished and characterised by atomic charges, electrophilicity and local softness indices. Dual-descriptor plots indicate an orbital σ hole on the halogen similar to the electrostatic σ hole manifested in the molecular electrostatic potential. 
The predicted high halogen-bond-acceptor affinity of N-heterocyclic carbenes was evidenced in the highest complexation energy for the hitherto unknown CF3I⋅NHC complex. The dominant NOCV orbital represents an electron-density deformation according to a n→σ*-type interaction. The characteristic signal found in the reduced density gradient versus electron-density diagram corresponds to the non-covalent interaction between contact atoms in the NCI plots, which is the manifestation of halogen bonding within the NCI theory. The unexpected C[BOND]X bond strengthening observed in several cases was rationalised within the molecular orbital framework.
The divergent reactivity of a transient titanium neopentylidyne, (PNP)Ti[triple bond, length as m-dash]CtBu (A) (PNP = N[2-PiPr2-4-methylphenyl]2-), that exhibits competing dehydrogenation and dehydroalkoxylation reaction pathways in the presence of acyclic ethers (Et2O, nPr2O, nBu2O, tBuOMe, tBuOEt, iPr2O) is presented. Although dehydrogenation takes place also in long-chain linear ethers, dehydroalkoxylation is disfavoured and takes place preferentially or even exclusively in the case of branched ethers. In all cases, dehydrogenation occurs at the terminal position of the aliphatic chain. Kinetics analyses performed using the alkylidene-alkyl precursor, (PNP)Ti[double bond, length as m-dash]CHtBu(CH2tBu), show pseudo first-order decay rates on titanium (kavg = 6.2 +/- 0.3 [times] 10-5 s-1, at 29.5 +/- 0.1 [degree]C, overall), regardless of the substrate or reaction pathway that ensues. Also, no significant kinetic isotope effect (kH/kD [similar] 1.1) was found between the activations of Et2O and Et2O-d10, in accord with dehydrogenation (C-H activation and abstraction) not being the slowest steps, but also consistent with formation of the transient alkylidyne A being rate-determining. An overall decay rate of (PNP)Ti[double bond, length as m-dash]CHtBu(CH2tBu) with a t1/2 = 3.2 +/- 0.4 h, 
across all ethers, confirms formation of A being a common intermediate. Isolated alkylidene-alkoxides, (PNP)Ti[double bond, length as m-dash]CHtBu(OR) (R = Me, Et, nPr, nBu, iPr, tBu) formed from dehydroalkoxylation reactions were also independently prepared by salt metatheses, and extensive NMR characterization of these products is provided. Finally, combining theory and experiment we discuss how each reaction pathway can be altered and how the binding event of ethers plays a critical role in the outcome of the reaction.
The transient titanium neopentylidyne, [(PNP)Ti≡CtBu] (A; PNP–≡N[2-PiPr2-4-methylphenyl]2–), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η2-H2C═CH2)(CH2tBu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C4–C6 to form isolable α-olefin complexes of the type, [(PNP)Ti(η2-H2C═CHR)(CH2tBu)] (R = CH3 (2), CH2CH3 (3), nPr (4), and nBu (5)). Complexes 1–5 can be independently prepared from [(PNP)Ti═CHtBu(OTf)] and the corresponding alkylating reagents, LiCH2CHR (R = H, CH3(unstable), CH2CH3, nPr, and nBu). Olefin complexes 1 and 3–5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including 1H–31P HOESY, and in the case of the α-olefin adducts 2–5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C–H coupled and decoupled 1H–13C multiplicity edited gHSQC, 1H–31P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C7 and C8) are also dehydrogenated by A to form [(PNP)Ti(η2-H2C═CHnPentyl)(CH2tBu)] (6) and [(PNP)Ti(η2-H2C═CHnHexyl)(CH2tBu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with D2C═CD2 with concomitant release of H2C═CH2. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH2tBu)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d12 can be dehydrogenated to 4 and 4-d12 with comparable rates (KIE = 1.1(0) at ∼29 °C). 
Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N2O, organic azides (N3R; R = 1-adamantyl or SiMe3), ketones (O═CPh2; 2 equiv) and the diazoalkane, N2CHtolyl2. For complexes 3–7, oxidation with N2O also liberates the α-olefin.
We report the first mononuclear three-coordinate vanadium(II) complex [(nacnac)V(ODiiP)] and its activation of N2 to form an end-on bridging dinitrogen complex with a topologically linear V(III)N2V(III) core and where each vanadium center antiferromagnetically couples to give a 
ground state singlet with an accessible triplet state as inferred by HFEPR spectroscopy. In addition to investigating the conversion of N2 to the terminal nitride (as well as the microscopic reverse process), we discuss its similarities and contrasts to the isovalent d3 system, [Mo(N[tBu]Ar)3], and the S = 1 system [(Ar[tBu]N)3Mo]2(μ2-η1:η1-N2).
A quantitative analysis of the steric effect of aliphatic groups was carried out from first principles. An intuitive framework is proposed that allows the separation and straightforward interpretation of two contributors to the steric effect: steric strain and steric shielding (hindrance). When a sterically demanding group is introduced near a reactive center, deformation of its reactive zone will occur. By quantifying this deformation, a convincing correlation was established with 
Taft's steric parameters for groups of typical size, supporting the intuitive image of steric shielding; bulky groups slow down the reaction by limiting the accessibility of the reactive centre. On the other hand, the strong initial repulsion between the reactant and the substrate by means of the filled-filled orbital interaction results in the deformation of the substrate as well as a less stabilizing reaction zone, which are the manifestations of the steric strain. We thus conclude that both steric strain and steric hindrance can be derived from the Pauli repulsion evolving between the reactants in the course of the reaction.
Electronic and steric factors that control the bond-stretch isomerism phenomenon in 2-chalcogen-trimetallabicyclo[1.1.0]butane systems have been investigated using quantum chemical calculations. Beside the short-bond and long-bond extremes we found and characterized a third, stable conformer with significantly elongated central bond and anti arrangement of the bridgehead substituents. Electronic structures of the isomers have been described and interpreted in details within the framework of MO theory. Germanium analogues of trisilirene have been predicted to also react with sulphur to form the corresponding 2-thia-trimetallabicyclo[1.1.0]butane analogues. 
The systematic analysis of the relative stability dependence on the ring composition as well as the bridgehead substituents revealed that the hitherto unknown anti isomer is a promising synthetic target in germanium bridged bicyclo compounds with bridgehead substituents containing carbon contact atom.
The argentate trinuclear cluster Ag3(μ2-3,5-(CF3)2PyrPy)3 (3,5-(CF3)2PyrPy = 2,2′-pyridylpyrrolide– ligand) catalytically promotes the insertion of the carbene of ethyl diazoacetate at room temperature into the C–H bond of a series of alkanes ranging from ethane to hexane, as well as branched and cyclic hydrocarbons. In addition to experimental studies, we also present theoretical studies elucidating the mechanism to C–H activation and functionalization by the transient silver carbene monomer (3,5-(CF3)2PyrPy)Ag(CHCO2Et). On the basis of DFT studies, formation of the silver carbene complex was found to be rate-determining for alkane substrates such as ethane and propane. On the other hand, DFT studies on methane, a substrate that we failed to activate, 
revealed that carbene insertion into the C–H bond was overall rate-determining. Theoretical analysis of charge flow also shows that the change from separated reagents to the TS involves charge flow from alkane to the silver carbene carbon with the bridging H behaving as a conduit. KIE studies using cyclohexane as a substrate suggest that the product-determining step involves only modest C–H bond lengthening, which can be also represented as a very early transition state with respect to C–H insertion of the carbene.
An experimental and theoretical DFT study was carried out on the solution behavior in [D7]DMF for bis-chelate complex [Pd(L)2](BF4)2·2CH3CN (L = 4-phenyl-1-(2-picolyl)-1,2,3-triazole). In structure of [Pd(L)2]2+, the central square–planar palladium(II) cation is trans-chelated by two L substrates, each through the pyridine and the triazole N2 nitrogen atoms, forming two six-membered metallacycles. These can adopt boat-like conformations anti-trans-[Pd(L)2]2+ and syn-trans-[Pd(L)2]2+ in which the picolyl methylene carbons are anti or syn, respectively, relative to the palladium coordination plane. In solution, the boat-to-boat inversion at both metallacycles takes place. The conformers are in a dynamic equilibrium, which was monitored by variable-temperature (VT) 1H NMR spectroscopy in the temperature range of 223–353 K. The equilibrium lies on the side of the anti-trans-[Pd(L)2]2+ conformer and the corresponding reaction enthalpy and entropy is estimated to be 0.6 ± 0.5 kcal mol−1 and 0.8 ± 1 cal mol−1 K−1, respectively. From the full-line-shape analysis of resonances in the VT 1H NMR spectra,
the activation enthalpy and activation entropy was determined to be 13.0 ± 0.4 kcal mol−1 and 2.7 ± 1.6 cal mol−1 K−1, respectively. The activation entropy close to zero suggests a nondissociative mechanism for the isomerisation. DFT investigation revealed that the isomerisation proceeds through a one step mechanism with a barrier of 11.40 kcal mol−1. The structures of the syn and anti conformers as well as that of the transition state were characterized. Energy decomposition analysis was carried out in order to explore the origins of the stability difference between the syn and anti isomers.
We demonstrate that a titanium-carbon multiple bond, specifically an alkylidyne ligand in the transient complex, (PNP)Ti[triple bond, length as m-dash]CtBu (A) (PNP- = N[2-P(CHMe2)2-4-methylphenyl]2), can cleanly activate 
methane at room temperature with moderately elevated pressures to form (PNP)Ti[double bond, length as m-dash]CHtBu(CH3). Isotopic labeling and theoretical studies suggest that the alkylidene and methyl hydrogens exchange, either via tautomerization invoking a methylidene complex, (PNP)Ti[double bond, length as m-dash]CH2(CH2tBu), or by forming the methane adduct (PNP)Ti[triple bond, length as m-dash]CtBu(CH4). The thermal, fluxional and chemical behavior of (PNP)Ti[double bond, length as m-dash]CHtBu(CH3) is also presented in this study.
The SEAr reaction was scrutinized using a quantitative, fragment-based interaction energy decomposition analysis (EDA) as implemented in the ADF program. 
The interaction energy between monosubstituted benzene derivatives and a model electrophile at the onset of the reaction was studied and decomposed into Pauli repulsion, electrostatic interaction and orbital interaction terms for a plane parallel to the molecular plane. As such experimentally observed selectivity patterns arise in the orbital interaction, stressing again the role of both electrophile and benzene derivative in determining the regioselectivity. Using the HSAB principle and MO theory, the orientation mechanism of the SEAr is further explored.
The transient titanium alkylidyne, (PNP)Ti≡CtBu (PNP = N[2-PiPr2-4-methylphenyl]2–), activates a C–H bond of ethane at room temperature, and a β-hydrogen of the resulting ethyl 
ligand is subsequently transferred to the adjacent alkylidene ligand to form an ethylene adduct of titanium. Treatment of the ethylene complex with two-electron oxidants such as organic azides results in extrusion of ethene concomitant with formation of a mononuclear titanium imido complex.
We report that 1-(2-picolyl)-1,2,3-triazole (click triazole) forms stable complexes with transition-metal ions in which the coordination involves the triazole N2 nitrogen atom and the pendant 2-picolyl group. This is exemplified by model compound 1-(2-picolyl)-4-phenyl-1H-1,2,3-triazole (Lx) and its complexes with transition-metal ions of PtII, PdII, CuII, RuII, and AgI. The coordination was investigated experimentally and theoretically. Ligand Lx easily reacted at room temperature with cis-[PtCl2(DMSO)2], [Pd(CH3CN)4](BF4)2, CuCl2, [RuCl(μ-Cl)(η6-p-cymene)]2, and AgNO3 to give stable chelates [PtCl2Lx] (1), [Pd(Lx)2](BF4)2 (2), [CuCl2(Lx)2] (3), [RuCl(η6-p-cymene)Lx]OTf (4), and [Ag2(Lx)2(NO3)2] (5), respectively, in 60−98% yield. The structures of 1−5 were unambiguously confirmed by NMR spectroscopy and single-crystal X-ray diffraction analysis. Density functional theory calculations were carried out in order to theoretically investigate the stabilization factors in 1−5. A comparison of the chelating properties of ligand Lx was made with structurally similar and isomeric 1-(2-aminoethyl)-substituted 1,2,3-triazole (Ly) 
and 4-(2-aminoethyl)-substituted 1,2,3-triazole (Lz). The complexation affinity of Lx was attributed to π-back-donation from the metal to the pendant pyridine side arm, whereas the stability of the complexes involving Ly and Lz mainly originates from efficient π-back-donation to the triazole ring.
The thermodynamic and kinetic stability of bis(diisopropylamino)cyclopropenylidene and related molecules were investigated by quantum chemical methods. The main stabilizing factor in the amino-substituted cyclopropenylidene is the significant π-electron shift from the amino substituents, via a nonaromatic ylidic structure; however, its successful synthesis is based on its high kinetic stability. 
The silicon and germanium analogues show lower stabilization than the synthesized carbene. However, when investigating their reactions, we found that both amino-disubstituted three-membered silylene and germylene have considerable kinetic stability.
The photonucleophilic aromatic substitution reactions of nitrobenzene derivatives were studied by ab initio and Density Functional Theory methods. The photohydrolysis is shown to proceed via an addition−elimination mechanism with two intermediates, except in the case of a chlorine leaving group. 
Depending on the substituents, the addition step, the elimination step, or the radiationless transition is the rate-determining process. The solvent effect on the SN2 Ar* reactions was evaluated by a continuum model. Next, the regioselectivity of the addition step is investigated within the framework of the so-called spin-polarized conceptual density functional theory. It is shown that the preference observed for the meta or para (with respect to the NO2 group) pathways in the addition step can be predicted by using the spin-polarized Fukui functions applied for the prereactive π-complex.
The substituent migration on the X2Y rings (X, Y=C, Si, Ge) was studied by theoretical method with silyl and hydrogen substituents and it was found that all the reactions (with the exception of cyclopropene) proceed in a two-step mechanism via a stable intermediate. The rate determining step of the reaction is the first step. The barrier of the second step is small and the energy of the intermediate is close to that of the reactant. Both the first transition state (T1) and the intermediate (I) are of monobridge structures of different types. Since the intermediate bridge structure is almost as stable as the product, it may be observed in the substituent migration reactions.
The geometry and the energetic aspects of the stable isomers of trimetallenes (X2YR4: X, Y = Si, Ge) were investigated, and some exotic di- and monobridge structures were found. 
Many of the bridge structures were identified as stable intermediates or transition states during the two-step substituent migration reactions. The effect of bulky substituents on the stability and geometry of the monobridge structures was studied. The bulkier bridging substituents cause a larger deviation from the ideal bridge structure. SiH3 and Si(SiH3)3 substituents confer a direct benefit on the bridge geometry, in spite of their bulkiness, via electronic effects. According to these results and considering the electronic effect of the Si(SiCnHm)3-type substituents, there is hope to synthesize bridge silicon compounds with bulky substituents.
The regioselectivity of ring-forming radical reactions is investigated within the framework of the so-called spin-polarized conceptual density functional theory. Two different types of cyclizations were studied. First, a series of model reactions of alkyl- and acyl-substituted radicals were investigated. Next, attention was focused on the radical cascade cyclizations of N-alkenyl-2-aziridinylmethyl radicals (a three-step mechanism). In both of these reactions, the approaching radical (carbon or nitrogen centered) adds to a carbon−carbon double bond within the same molecule to form a radical ring compound. 
In this process, the number of electrons is changing from a local point of view (a charge transfer occurs from one part of the molecule to another one) at constant global spin number Ns (both the reactant and the product ring compound are in the doublet state). It is shown that the experimentally observed regioselectivities for these ring-closure steps can be predicted using the spin-polarized Fukui functions for radical attack, (r).
Regioselectivity of the photochemical [2 + 2] cyclo-addition of triplet carbonyl compounds with a series of ground state electron-rich and electron-poor alkenes, the Paterno-Büchi reaction, is studied. Activation barriers for the first step of the triplet reaction are computed in the case of the O-attack. Next, the observed regioselectivity is explained using a series of DFT-based reactivity indices. In the first step, we use the local softness and the local 
HSAB principle within a softness matching approach, and explain the relative activation barriers of the addition step. In the final step, the regioselectivity is assessed within the framework of spin-polarized conceptual density functional theory, considering response functions of the system’s external potential v, number of electronsN and spin numberNs, being the difference between the number ofα andβ electrons in the spin-polarized system. Although the concept of local spin philicity, introduced recently within this theory, appears less suited to predict the regioselectivity in this reaction, the correct regioselectivity emerges from considering an interaction between the largest values of the generalized Fukui functionsfss on both interacting molecules.
