Tolman's electronic parameters for divalent carbon(0) compounds

The complexes [D-Ni(CO)₃] and [D-RhCl(CO)₂] have been calculated with DFT methods where D is a strong carbon or phosphorus donor ligand. The focus of the work is on the estimate of the donor strength of divalent carbon(O) compounds CL₂ compared to common carbon(II) ligands (N-heterocyclic carbenes, NHC), phosphines, and several ligands recently introduced. Tolman's electronic parameters are derived by a fit procedure to experimental data to enable better comparison of different donor strength scales. It turns out that carbon(O) ligands CL₂ with L = PR3 (carbodiphosphoranes) or NHC (carbodicarbenes) are much stronger donors than carbene and phosphine ligands. Besides the calculation of known substitution patterns, several new ligands of this class are proposed that might be valuable in the search for new ligands with adjustable donor properties.