Nonclassical Metal Carbonyls: Appropriate Definitions with a Theoretical Justification

MP2 calculations show that the D∞h isoelectronic dicarbonyl complexes [M(CO)2]n (Mn=Rh−, Pd0, Cu+, Ag+, Au+, Zn2+, Cd2+, Hg2+) depicted in structure 1 can be classified as classical or nonclassical depending on whether the metal–carbon bond lengths decrease or increase when weak, anionic ligands approach the metal centers.