Deconvoluting the Role of Electrostatics in Metal Carbonyl Bonding: Dipole Moments and Energy Decomposition Analysis of Late Transition Metal Pincer Complexes

Abstract

While the primary orbital interactions involved in the metal-carbonyl bond are captured by the Dewar-Chatt-Duncanson model, the emerging consensus is that orbital polarization effects caused by electrostatic interactions also play a significant role. With reference to the carbonyl stretching frequencies of a large computational data set of symmetric platinum group metal pincer complexes, we herein show that the latter can be interpreted by reference to the well-established vibrational Stark effect using the dipole moment of the supporting metal fragment as a proxy for the electric field being projected over the carbonyl ligand. Specifically, when this dipole moment is used in combination with the orbital interaction energy for π-backbonding, determined by energy decomposition analysis, carbonyl stretching frequencies are reproduced with much greater statistical significance (R2 = 0.934 vs 0.798) and lower RMSDs (22 vs 39 cm-1) than with the orbital term only. Deconvolution of the metal-carbonyl interaction in this way is physically meaningful and provides a conceptually simple way to explain trends in υ(CO) and account for the existence of nonclassical carbonyl complexes, in which υ(CO) > 2143 cm-1