Revisiting $\pi$ Backbonding: The Influence of $d$ Orbitals on Metal-CO Bonds and Ligand Red Shifts

TL;DR

This study reevaluates the role of d orbitals in metal-CO bonds, showing that d orbitals significantly enhance stability but only slightly influence CO vibrational shifts, challenging traditional views of π backbonding.

Contribution

It demonstrates that metal d orbitals are crucial for stability but have limited impact on CO red shifts, revising the classical understanding of π backbonding in metal carbonyls.

Findings

Metal d orbitals greatly enhance complex stability.

CO red shift occurs even without metal-CO π bonds.

Traditional π backbonding model is challenged.

Abstract

The concept of $\pi$ backbonding is widely used to explain the complex stabilities and CO stretch frequency red shifts of transition metal carbonyls. We theoretically investigate a non-transition metal 18-electron carbonyl complex (Mg(CO)$_8$) and find a pronounced CO red shift without metal-carbon $\pi$ bonds. Moreover, we use truncated basis sets on the "honorary" and true transition metals Ca and Ti in Ca(CO)$_8$ and [Ti(CO)$_8$]$^{2+}$ complexes to probe the influence of $d$ functions on carbonyl complex stability, C-O bond strength, metal-to-ligand charge transfer and bond order compared to hypothetical complexes without metal-$d$ contributions. We find that the occurrence of metal-ligand $\pi$ bonds through metal $d$ functions greatly enhances the complex stabilities on one hand but only slightly affects the CO red shift on the other hand. This does not correspond to the classical rationalization of transition metal-CO bonds as synergistic $\sigma$ donation/$\pi$ backdonation.