Intuitive Quantifiers of Charge Flows in Coordinate Bonding

ETS–NOCV charge and bond energy analyses have been carried out for a broad range of transition-metal carbonyl complexes L–[M], comprising different ligand classes, transition metals, and coordination geometries. The resulting electronic redistributions are visually assigned to σ donation, π backbonding, and related interactions. We propose a Hirshfeld partitioning of these electronic redistributions to afford the corresponding charge flow contributions Δqσ, Δqπ, etc. Taken together, a detailed picture of the dative bonding arises, in terms of both energetics and the extent of σ-electron donation and π-electron backbonding. The charge flows Δqσ and Δqπ appropriately quantify trends in the ligand σ-donor and π-acceptor abilities and are transferable across the transition-metal complexes studied and thus promise to be suitable descriptors for ligand knowledge bases. As a case in point, the TEP is well reproduced by the calculated νCO(A1) frequencies and is 3 times more strongly affected by Δqσ than by Δqπ, with an additional modest steric influence. Further, empirical relationships are derived among the charge flows Δqσ and Δqπ, the (L)­W­(CO)5 carbonyl stretching frequencies, and the ligand’s steric volume %Vbur, which allow estimating the σ-donor and π-acceptor abilities of phosphines from experimental observables. On the other hand, direct Cl:→L−σ* interactions are identified in several cis-(L)­Ir­(CO)2Cl complexes, which compromises the use of these species as experimental probes for ligand parameters.