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 [M-2(CO)(2)](2+) and [M-2(CO)(6)](2+) (M = Ni, Pd, and Pt) were studied using the B3YLP density functional theory. [M-2(CO)(2)](2+) cations have two stable conformations; one is the GO-bridged structure in the D-2h symmetry for M = Ni, and C-2 nu symmetry for Pd and Pt, and the other is the linear structure in the D-infinityh symmetry. The former is more stable than the later for each of M = Ni, Pd, and Pt. The GO-bridged [M-2(CO)(2)](2+) are characterized by sigma -donation and pi back-donation which lead to longer C-O bond lengths of (R(CO)= 1.130-1.137 Angstrom) and lower CO vibrational frequencies of (nu (CO) = 2055-2100 cm(-1)) than those of free CO [R(CO) = 1.128 Angstrom, nu (CO) = 2143 cm(-1)], while the nu (CO) is much higher than the values of the typical bridged metal carbonyls (1750-1850 cm(-1)). The linear [M-2(CO)(2)](2+) have shorter R(CO)s (1.112-1.115 Angstrom) and higher nu (CO)s (2230-2260 cm(-1)) than those of free CO owing to the reduced pi back-donation. [M-2(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 degrees with respect to each other. The R(CO)s (1.115-1.119 Angstrom) are shorter and nu (CO)s (2186-2240 cm(-1)) are higher than those of free CO. A transition state was found for [M-2(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-2 nu symmetry. The R(CO)s for bridging CO are longer than those for terminal COs. The calculated nu (CO)s for the GO-bridged [Pd-2(CO)(2)](2+) and minimum [Pt-2(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. (C) 2001 Elsevier Science B.V. All rights reserved.