“…While it is difficult to separate kinetic and thermodynamic contributions to complex formation, the nature of the Ln−M bonding is highly contingent upon the nucleophilicity of the transition metal in the carbonylate anion, [M(CO) y ] n - . Specifically, the relative Lewis basicities of M, the CO ligands, and the solvent determine the type of interaction between Ln and M. Direct Ln−M bonded systems ( I , Chart ) are yielded when the transition-metal center is the most nucleophilic constituent (i.e., electron density resides on M). ,10a,b A solvent-separated ion pair ( II ) results when the electron-donating ability of the solvent exceeds that of [M(CO) y ] n - . ,6c,11c, If the carbonyl oxygens are more Lewis basic than M, then an isocarbonyl bridge ( IIIa − c ) to Ln is formed. 2c,6d, Low polarity or nonnucleophilic solvents can also favor the formation of an isocarbonyl even when [M(CO) y ] n - is a weak nucleophile. 9a, While the two types of isocarbonyl interactions η 2 , μ 2 -CO ( μ -CO; IIIa ) 2c,6d,9a,c,e,13 and η 2 , μ 3 -CO 9b,d ( IIIb ) are known, Ln−M examples of η 2 , μ 4 -CO bridges ( IIIc ) have not been reported …”