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.