Hartree—Fock—Slater-LCAO studies of the acetylene—transition metal interaction. III. Binding to mono- and dinuclear Ni complexes with carbonyl and isocyanide ligands

The bonding of C₂H₂ in the nickel complexes [π-(C₂H₂)Ni(CO)₂], [π-(C₂H₂)Ni(CNH)₂] and [μ₂-(C₂H₂) {NI(CNH)₂}₂] has been studied by the non-empirical self-consistent Hartree—Fock—Slater-LCAO method. Calculations have been performed, not only for these complexes, but also for free C₂H₂ and its substituted analogues, propyne, but-2-yne and hexafluorobut-2-yne, and for the mono- and dinuclear complex fragments, [Ni(CNH)₂] and [{NI(CNH)₂}₂], which interact with the C₂H₂ molecule. The main difference with nickel surfaces, which have analogous π and μ₂ adsorption sites for C₂H₂ that we have studied by the same method, is the much stronger 3d, 4s, 4p hybridization induced by the CNH and CO ligands. In the bis(diisocyanide—nickel) complex these hybrids form a rather broad dsp “band”. The low lying occupied levels in this “band” cause a strong interaction with the filled acetylene π_u orbitals which dominates the π to metal donation effect. In the mononickel—acetylene complex this donation does occur, however, just as for acetylene adsorbed on nickel surfaces (in combination with the more prominent metal to π^* back donation which we find in all cases). A linear relation has been derived between the CC stretch frequencies in coordinated C₂H₂ molecules and the calculated CC overlap populations. Substitutio of the H atoms in (free) C₂H₂ by CH₃ or CF₃ groups has almost no effect on the CC overlap population and force constant.