Optimized structures and energetics of mixed-metal binuclear carbonyl ions MCr(CO)n+ (M=Ti–Fe, n=4–6) by density functional method

Abstract

Optimized structures and energetics of coordinatively unsaturated mixed-metal binuclear carbonyl ions MCr(CO)n+ (M=Ti, V, Cr, Mn and Fe; n=4–6), previously obtained by the laser ablation-molecular beam (LAMB) experiment, have been studied by the density functional method with the gradient correction at the Becke–Perdew level. The positive charge lowers the metal d orbitals in energy, weakens their interaction with the carbonyl 2π∗ orbitals, and strengthens that with the carbonyl 5σ orbitals in comparison with the neutral carbonyls. The predominance of the σ-donation over the π∗-back donation increases from M=Ti to Fe in this order and with the increase of n for each M. These are reflected in the M–C and the C–O bond lengths. M-dependent variation in the M–Cr bond distance, significant in diatomic ions MCr+, is largely lost in MCr(CO)5+ and MCr(CO)6+. Anticorrelation is found between the C–O bond lengths on the M side and those on the Cr side. The relative stability inferred from the relative abundance of the ions in the LAMB experiments is qualitatively explained by comparing the M+–Cr binding energy with the successive CO bond dissociation energies of MCr(CO)6+.