Noncovalent Bonding of Group 4 Metals

Analysis of numerous crystal structures from the Cambridge Structural Database (CSD) suggests bonding interactions between group 4 metal atoms and Lewis bases. DFT calculations consider the bond between the central group 4 metal atom of MX₂Y₂ (M = Ti, Zr, Hf; X = F, Cl, Br, I; Y = N = C(CH₃)₂) and Lewis base NH₃. Strong interactions, bordering on covalency, are observed in all cases that rely in part on the σ-hole on the M atom. Interaction energies lie between 25 and 30 kcal/mol. The bond grows stronger as M or X is enlarged, but this growth is gradual. Because of the larger deformation energies required for the Ti Lewis acid to adjust its structure to the incoming nucleophile, the binding energies follow the order Ti < Zr < Hf and occupy the range between 15 and 21 kcal/mol. The bonding of Ti is dominated by electrostatics, but it is polarization that plays the larger role for Hf.