Why do CO molecules form strong bonds with metal atoms in carbonyls?

Molecules of CO

and NN have two π bonds. Since the two atoms in CO are different, this made CO much more reactive than nitrogen. Indeed, CO

forms many carbonyls with metal atoms or ions. For example, you have encountered some of the following carbonyls on the page of Heterogeneous catalysts

• Ni(CO)4
• Fe(CO)5
• Co2(u-CO)2(CO)6, (u-CO meaning CO bridged between two metal atoms)
• Mn2(CO)10
• Fe3(CO)12
• Co4(CO)12
• Rh4(CO)12
• CFe5(CO)15
• Rh6(CO)16
• Os6(CO)18

The study of metal carbonyls started with the discover of ferrocene Fe(C₅H₅)₂ and now hundreds if not thousands of metal carbonyls have been synthesized..

A sbond is formed between the carbon of CO and the metal atom. Such a bonding is very sensible if we consider the sp hybrid atomic orbitals of carbon being used in this case. Since there are two electrons in this orbitals of CO, the metal atom gains at least some fraction of electron due to the formation of this bond.

The empty antibonding pi orbital π∗ of CO has the right symmetry and orientation to receive the back-donated electron from the filled d-orbitals of the metal atom. The back donation reinforce the sigma bond, and vice versa. This type of bonding has been called the synergic bonding mechanism by Cotton and Wilkinson in their Advanced Inorganic Chemistry. A diagram showing this type of bonding scheme is shown on page 159 in Inorganic Chemistry by Swaddle.