Semicoordinate versus σ-Hole Bonding of Group 10 Metal Atoms in a Square Planar Motif

Complexes are formed by a group 10 metal (Pt, Pd, Ni) in a square planar M(S₂CH)₂ unit with a series of small AF_n molecules containing A atoms from groups 14–17. In many cases, A lies directly above the M, while it tends to migrate toward the liganding S atoms, particularly as the electrostatic potential above M switches sign from negative to positive or as the σ-hole on the A atom grows deeper. In most complexes, there is an element of σ-hole bonding involving charge transfer from the M d_z2 to a σ*(AF) antibonding orbital. However, this component is dwarfed by transfer in the opposite direction, from the A lone pair to a vacant M p_z orbital, leading to its designation as a semicoordinate bond. Interaction energies lie in the range between 4 and 11 kcal/mol. In many cases, Ni and Pt have similar bond strengths, with that of Pd being slightly weaker. The interaction energies of various A atom classes, whether halogen, chalcogen, pnicogen, or tetrel, are rather similar to one another, although there is a clear tendency for the binding to strengthen as the A atom grows in size.