BOND STRENGTHS IN 1ST-ROW-METAL CARBONYL ANIONS

Energy-resolved collision-induced dissociation of metal carbonyl anions M(CO)n-(M = V, Cr, Mn, and Co) is used to determine sequential metal-carbonyl bond energies. The derived thermochemistry can be used in conjunction with literature measurements of neutral chromium carbonyl electron affinities to provide (CO)(n)Cr-CO bond strengths, which are compared to previous experimental determinations. Literature observations of ligand exchange reactions are used to derive other metal-ligand bond strengths. Analysis of the present data and previous values for the iron and nickel carbonyl anions shows that most metal-carbonyl ligand bond strengths are ca. 40 kcal/mol, which is apparently the intrinsic metal-carbonyl bond strength in anions. Several of the values for D[M--2CO] are significantly lower than average; these bond strengths correlate with the energy needed to promote the metal anion into a state suitable for bonding. The electronic promotion is apparently complete after the addition of two carbonyl ligands. Comparisons of isoelectronic species confirm the general order of metal-carbonyl bond strengths, anion greater-than-or-equal-to neutral greater-than-or-equal-to cation.