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 nu(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 sigma bonds having a significant electrostatic component. There is relatively little M-->CO pi backbonding. However, despite the similarity in nu(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.