Copper(I) and silver(I) carbonyls. To be or not to be nonclassical

The traditional, or classical, picture of M–CO bonding is not a good model for more than 200 metal carbonyl species with average ν(CO) values greater than 2143 cm−1. These “nonclassical” complexes, exemplified in this Perspective by the cations Cu(CO)n+ and Ag(CO)n+ (n = 1–4), possess M–CO interactions that are best described as M←CO σ bonds having a significant electrostatic component. There is relatively little M→CO π backbonding. However, despite the similarity in ν(CO) values, homologous Cu(CO)n+ and Ag(CO)n+ complexes behave differently when subjected to perturbations such as stretching the metal–carbon bonds or adding a pair of weak ligands to the metal center.