Reassessment of the electronic and molecular structure, bonding, and stability of zerovalent nickel acetylene complexes by the density functional method

The density functional theory (DFT) method has been used to study the electronic and molecular structure of mono-, di-, and trinuclear zerovalent nickel complexes containing acetylene and sigma-donor ligands. The calculations were carried out; for the model compounds (C2H2)(2)Ni (10) and (C2H2)Ni(PH3)(2) (12) with quasi-tetrahedral (10a, 12a) and planar (10b, 12b) conformations as well as for (C2H2)(3)Ni-2 (14) and (C2H2)(4)Ni-3 (23). Rotational preference of complexes 10 and 12 is discussed on the basis of the 18 VE rule, relative energies, and natural bond orbital (NBO) population analyses. Optimized geometries and calculated IR and NMR properties are compared with known experimental data. It is shown how the effective hack-bonding into acetylene in-plane pi(parallel to)* MO(s) accounts for rotational preference of 10 and 12 as well as for the main features of molecular geometry of polynuclear Ni(0) compounds. Binding energies (BEs) of acetylene in 10a and 12b are calculated at; the DFT, HF, MP2-MP4, CCSD, and CCSD(T) levels and compared to those of ethylene in (C2H4)(2)Ni (17), (C2H4)Ni(C2H2) (18), and (C2H4)Ni(PH3)(2) (19) as well as to those of CO in Ni(CO)(x), x = 4 (20), 3 (21), 2 (22). It turns out that with respect to 10a the bridging acetylene of 14 is bound almost 2 times stronger. Calculated BEs together with energies of association reactions L' + NiL2 --> NiL2L' (L, L' = CO, PH3, C2H2) and L + NiL3 --> NiL4 (L = CO, PF3, PMe3, PH3, C2H2) as well as of the exchange reaction Ni(CO)(4) + 2C(2)H(2) --> Ni(C2H2)(2) + 4CO are used in the discussion of thermodynamic and kinetic stability of the formally two-coordinated bis(alkyne)Ni compounds.