Answer to Question #122153 in Inorganic Chemistry for karthik

Question

Calculate the valance electron concentration per metal atom in each of the following structures with a general formula M3(CO)12; (eq = equatorial CO; ax = axial CO group); indicate the electron contribution from each group.

Answer 1

The 18-electron rule and its extensions to polynuclear systems apply also in the presence of carbonyl ligands although conceptual distinctions are needed. For terminal COs, the metal-carbon σ bond is due to the two-electron donation from the ligand, whereas two or four electrons used in back-donation and giving rise to multiple metal-carbon π bonding are counted as if they are metal lone pairs. In systems with bridging COs, the formal electron counting does not change (two electrons per CO) despite the presence of pairwise equivalent metalcarbon σ bonds.