Syntheses and Crystal Structures of [Hg{C(PPh3)2}2][Hg2I6] and [Cu{C(PPh3)2}2]I and Comparative Theoretical Study of Carbene Complexes [M(NHC)2] with Carbone Complexes [M{C(PH3)2}2] (M = Cu+, Ag+, Au+, Zn2+, Cd2+, Hg2+)

The synthesis and X-ray structure analysis of the carbodiphosphorane (CDP) complexes [Hg{C(PPh3)2}2][Hg2I6] and [Cu{C(PPh3)2}2]I are reported. The cations [Hg{C(PPh3)2}2]2+ and [Cu{C(PPh3)2}2]+ have approximately linearly coordinated metal atoms. Quantum chemical calculations of model compounds bearing N-heterocyclic carbene (NHC) ligands, [M(NHC)2] and [M{C(PH3)2}2] (M = Cu+, Ag+, Au+, Zn2+, Cd2+, Hg2+), have been carried out at the BP86/TZ2P level. The metal–ligand bonds are very strong, and the bond dissociation energies exhibit a V-shaped trend for first-, second-, and third-row transition metals: Ag+ + + and Cd2+ 2+ 2+. The investigation of the bonding situation in the complexes using an energy decomposition analysis shows that the metal–ligand bonding comes mainly from electrostatic attraction. Inspection of the orbital interactions shows that the Mq←(NHC)2 and Mq←{C(PH3)2}2 σ donation provides between 65 and 75% of the total orbital interactions ΔEorb. The contribution of the Mq→(NHC)2 and Mq→{C(PH3)2}2 π back-donation is very weak. The nature and strength of the donor–acceptor bonds of the two-electron donor ligand NHC and the four-electron donor ligand CDP with the group 11 and group 12 metal cations are very similar.