Quantum chemical calculations at the DFT level have been carried out for the title compounds. The equilibrium geometries and bond dissociation energies are reported. The nature of the bonding between the metal and the π ligands ethylene and acetylene has been investigated by means of an energy partitioning analysis. The nature of the metal−ligand interactions is not very different from each other in the donor−acceptor complexes (CO)5TM−C2H x (TM = Cr, Mo, W), (CO)4TM−C2H x (TM = Fe, Ru, Os), and TM+−C2H x (TM = Cu, Ag, Au). The metal−C2H x bonds have a slightly more electrostatic than covalent character. The covalent bonding comes mainly from the metal ← ligand σ donation and the metal → ligand π∥ in-plane back-donation. The contributions from the out-of-plane π⊥ and δ orbitals are negligible. The main difference of the bonding interactions in the metallacyclic compounds Cl4TM−C2H x (TM = Cr, Mo, W) is that they are clearly more covalent than electrostatic. The covalent interactions come also mainly from a1(σ) and b2(π∥) interactions. The a2(δ) orbital interactions are negligible but the interactions of the out-of-plane π orbitals in the acetylene complexes Cl4TM−C2H2 contribute ∼11% to the total orbital term.

The nature of the metal-ligand bonding in ferrocene and bis(benzene)chromium has been analyzed with the help of an energy partitioning scheme using the results of DFT calculations. The bonding analysis suggests that the Cr-Bz 2 bond is 37.9% electrostatic and 62.1% covalent. The binding interactions in ferrocene are predicted to be 51.1% electrostatic and 48.9% covalent if the charged species Fe 2+ and (Cp -) 2 are used as interacting fragments, while they are 45.0% electrostatic and 55.0% covalent if neutral Fe and Cp 2 in the triplet states are used. The largest contributions to the orbital interactions in bis(benzene)chromium come from the CrfBz 2 δ-back-donation, while the most important orbital contribution in ferrocene comes from the FerCp 2 π-donation. The larger contributions of the e 1g (π) orbitals in ferrocene are caused by better energy matching rather than better overlapping of the interacting orbitals.

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The use of different models based on experimental information about the observed level splitings, rotational constants, and far-infrared transition frequencies leads to different predictions on the equilibrium geometry for tetrahydrofuran. High-level ab initio calculations [coupled cluster singles, doubles (triples)/complete basis set (second order Moller-Plesset triple, quadrupole, quintuple)+zero-point energy(anharmonic)] suggest that the equilibrium conformation of tetrahydrofuran is an envelope C(s) structure. The theoretical geometrical parameters might be helpful to plan further microwave spectroscopic studies in order to get a physical interpretation of the measurements.

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We report on quantum chemical calculations at the DFT (BP86/TZP) and ab initio (CCSD(T)/III+) levels of the title compounds. The geometries, vibrational spectra, heats of formation, and homolytic and heterolytic bond dissociation energies are given. The calculated bond length of Cu-CN is in reasonable agreement with experiment. The theoretical geometries for CuNC and the other group 11 cyanides and isocyanides which have not been measured as isolated species provide a good estimate for the exact values. The theoretical bond dissociation energies and heats of formation should be accurate with an error limit of +/-5 kcal/mol. The calculation of the vibrational spectra shows that the C-N stretching mode of the cyanides, which lies between 2170 and 2180 cm(-)(1), is IR inactive. The omega(1)(C-N) vibrations of the isocyanides are shifted by approximately 100 cm(-)(1) to lower wavenumbers. They are predicted to have a very large IR intensity. The nature of the metal-ligand interactions was investigated with the help of an energy partitioning analysis in two different ways using the charged fragments TM(+) + CN(-) (TM = transition metal) and the neutral fragments TM(*) + CN(*) as bonding partners. The calculations suggest that covalent interactions are the driving force for the formation of the TM-CN and TM-NC bonds, but the finally formed bonds are better described in terms of interactions between TM(+) and CN(-), which have between 73% and 80% electrostatic character. The contribution of the pi bonding is rather small. The lower energy of the metal cyanides than that of the isocyanides comes from the stronger electrostatic interaction between the more diffuse electron density at the carbon atom of the cyano ligand and the positively charged nucleus of the metal.

Cyanides and isocyanides of first-row transition metal M(CN) (M=Sc-Zn) are investigated with quantum chemistry techniques, providing predictions for their molecular properties. A careful analysis of the competition between cyanide and isocyanide isomers along the transition series has been carried out. In agreement with the experimental observations, late transition metals (Co-Zn) clearly prefer a cyanide arrangement. On the other hand, early transition metals (Sc-Fe), with the only exception of the Cr(CN) system, favor the isocyanide isomer. The theoretical calculations predict the following unknown isocyanides, ScNC(3Delta), TiNC(4Phi), VNC(5Delta), and MnNC(7Sigma+), and agree with the experimental observation of FeNC(6Delta) and the CrCN(6Sigma+) cyanide. First-row transition metal cyanides and isocyanides are predicted to have relatively large dissociation energies with values within the range 80-101 kcal mol(-1), except Zn(CN), which has a dissociation energy around 50-55 kcal mol(-1), and low isomerization barriers. A detailed analysis of the bonding has been carried out employing the topological analysis of the charge density and an energy decomposition analysis. The role of the covalent and electrostatic contributions to the metal-ligand bonding, as well as the importance of pi bonding, are discussed.

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