Semicoordinate and halogen bonding to group 10 and group 8 metals

A series of square planar systems are constructed, placing a group 10 metal atom at the center, connected to two ditopic ligands by four M-S bonds. DFT calculations show that the metal can form a noncovalent bond with an approaching NH₃ ligand, with a strength that varies from 10.8 kcal mol^-1 for Ni, down to 1.8 kcal mol^-1 for Pt. This pattern conforms to the charge on the M which reverses from positive to negative in this same order. A XCCH molecule (X = I, Cl) can approach the metal system in a perpendicular configuration. Although this geometry suggests halogen bonding through electron donation from M to the X σ-hole, detailed scrutiny of the electronic structure shows the strongest element to be noncovalent semicoordinate bonding, involving charge transfer from X lone pairs to M. Other stable configurations place the XCCH unit parallel to the metal system, also held together by a semicoordinate bond. Group 8 metals form a shorter and stronger covalent bond with NH₃. While Os forms perpendicular arrangements with XCCH, Fe and Ru do not.