Energy analysis of metal–metal bonding in [RM–MR] (M = Zn, Cd, Hg; R = CH3, SiH3, GeH3, C5H5, C5Me5)

The metal–metal bonds of the title compounds have been investigated with the help of energy decomposition analysis at the DFT/TZ2P level. In good agreement with experiment, computations yield Hg–Hg bond distance in [H3SiHg–HgSiH3] of 2.706 Å and Zn–Zn bond distance in [(η5-C5Me5)Zn–Zn(η5-C5Me5)] of 2.281 Å. The Cd–Cd bond distances are longer than the Hg–Hg bond distances. Bond dissociation energies (-BDE) for Zn–Zn bonds in zincocene −70.6 kcal/mol in [(η5-C5H5)2Zn2] and −70.3 kcal/mol in [(η5-C5Me5)2Zn2] are greater amongst the compounds under study. In addition, [(η5-C5H5)2M2] is found to have a binding energy slightly larger than those in [(η5-C5Me5)2M2]. The trend of the M–M bond dissociation energy for the substituents R shows for metals the order GeH3 < SiH3 < CH3 < C5Me5 < C5H5. Electrostatic forces between the metals are always attractive and they are strong (−75.8 to −110.5 kcal/mol). The results demonstrate clearly that the atomic partial charges cannot be taken as a measure of the electrostatic interactions between the atoms. The orbital interaction (covalent bonding) ΔEorb is always smaller than the electrostatic attraction ΔEelstat. The M–M bonding in [RM–M–R] (R = CH3, SiH3, GeH3, C5H5, C5Me5; M = Zn, Cd, Hg) has more than half ionic character (56–64%). The values of Pauli repulsions, ΔEPauli, electrostatic interactions, ΔEelstat, and orbital interactions, ΔEelstat are larger for mercury compounds as compared to zinc and cadmium.