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.

Graphical abstractThe trend of the M–M bond dissociation energy in [RM–MR] 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).Figure optionsDownload full-size imageDownload as PowerPoint slide