Theoretical study of the monocarbonyls of first-row transition metal atoms

The results of density functional calculations on the most stable high-spin and low-spin states of MCO are given, where M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The ground states are found to be high spin for M=Sc, Ti, V, Cr, and Cu (2S+1=4, 5, 6, 7, and 2, respectively) and low spin for M=Mn, Fe, Co, and Ni (2S+1=4, 3, 2, and 1, respectively). From Sc to Cu, the M–CO binding energies with respect to ground state products are estimated to be 9, 16, 26, 13, −14, 14, 30, 54, and 19 kcal/mol. Where comparison with experiment is possible, the estimates are apparently too large by about 6 kcal/mol (FeCO), 13 kcal/mol (NiCO), and 12 kcal/mol (CuCO). The high-spin state MCO complexes with metal to the right of vanadium in the Periodic Table all have bent equilibrium geometries; all others are found to be linear. The calculated CO harmonic stretch frequencies generally overestimate the observed values, but follow a similar trend. The CO bond lengths, CO stretch frequencies, and metal–CO bond strengths all correlate well with the extent of π back donation. However, these correlations hold only within either the group of all high-spin states, or the group of all low-spin states. Thus, there are no simple trends in the calculated properties of ground state MCO complexes.