Geometry and electronic structure of binuclear metal carbonyl cations, [M₂(CO)₂]²⁺ and [M₂(CO)₆]²⁺ (M = Ni, Pd, Pt)

The geometries and electronic structures of group 10 metal binuclear carbonyl cations of [M₂(CO)₂]²⁺ and [M₂(CO)₆]²⁺ (M = Ni, Pd, and Pt) were studied using the B3YLP density functional theory. [M₂(CO)₂]²⁺ cations have two stable conformations; one is the CO-bridged structure in the D_2h symmetry for M = Ni, and C_2v symmetry for Pd and Pt, and the other is the linear structure in the D_ooh symmetry. The former is more stable than the later for each of M = Ni, Pd, and Pt. The CO-bridged [M₂(CO)₂]²⁺ are characterized by σ-donation and π back-donation which lead to longer C-O bond lengths of (R(CO) = 1.130-1.137 Å) and lower CO vibrational frequencies of (v(CO)= 2055-2100cm^-1) than those of free CO [R(CO)= 1.128 Å, v(CO)= 2143 cm^-1], while the v(CO) is much higher than the values of the typical bridged metal carbonyls (1750-1850 cm^-1). The linear [M₂(CO)₂]²⁺ have shorter R(CO)s (1.112-1.115 Å) and higher v(CO)s (2230-2260 c^m-1) than those of free CO owing to the reduced w back-donation. [M²(CO)₆]²⁺ cations have only one stable conformation (minimum) in the D_2d symmetry, which contains two essentially planar tricarbonyl metal units that are linked via a metal-metal bond about which they are twisted by 90.0° with respect to each other. The R(CO)s (1.115-1.119 Å) are shorter and v(CO)s (21862240 cm^-1) are higher than those of free CO. A transition state was found for [M₂(CO) ₆]²⁺ in which two carbonyls are symmetrically bridging onto two metals and the other four carbonyls are bound to each metal terminally with C_2v symmetry. The R(CO)s for bridging CO are longer than those for terminal COs. The calculated v(CO)s for the CO-bridged [Pd₂(CO)2]²⁺ and minimum [Pt₂(CO)₆] ²⁺ are in good agreement with available data given by IR and Raman spectroscopy for corresponding species. The bonding natures of M-C and C-O bonds are discussed in detail with reference to the molecular orbital energy diagrams, the results of population analysis, and the relativistic effects in metal atoms.