ENERGETICS AND ELECTRONIC-STRUCTURE OF CHROMIUM HEXACARBONYL

We investigate the molecular orbital self-consistent-field model of bonding in Cr(CO)6. The energetics and electron density are examined using a large range of tools. The change in density compared to a promoted 3dt2g6(1A1g) chromium atom and six CO molecules is primarily charge transfer from the t2g orbitals of chromium to the empty 2-pi*t2g orbital of the (CO)6 cage. This mixing is counterintuitive, as the largest increase in electron density is in the oxygen pi-orbitals. The restricted Hartree-Fock energy is actually repulsive compared to that of ground-state fragments by 111 kcal/mol. This energy change consists of +266 kcal/mol of fragment promotion energy, +67 kcal/mol of (CO)6 cage formation energy, -272 kcal/mol of electrostatic attraction, +359 kcal/mol of overlap repulsion between Cr and (CO)6, and finally -329 kcal/mol of orbital relaxation energy. Most of the relaxation energy is associated with the t2g HOMO-LUMO charge transfer. The sigma-electrons contribute to the bond energy primarily through electrostatic penetration, leading to a large electrostatic attraction between Cr and (CO)6, and not through mixing of CO sigma-orbitals with Cr empty valence orbitals.