Difference Between Carbonyl and Nitrosyl Complexes

The key difference between carbonyl and nitrosyl complexes is that carbonyl complexes contain -CO ligands, whereas nitrosyl complexes contain -NO ligands.

Coordination complexes are inorganic complexes containing a central atom or an ion (typically a metal ion) and surrounding molecules named ligands that are bound to the central atom or ion. The central atom or ion is usually known as the coordination center.

1. Overview and Key Difference
2. What are Carbonyl Complexes
3. What are Nitrosyl Complexes
4. Carbonyl vs Nitrosyl Complexes in Tabular Form
5. Summary – Carbonyl vs Nitrosyl Complexes

What are Carbonyl Complexes?

Carbonyl complexes are coordinate complexes consisting of a metal center and carbonyl ligands. The carbonyl ligands that we can find mainly in these complexes are -CO ligands. We can frequently use carbonyl ligands as starting materials for other coordination compounds as well because -CO ligands have no charge, and CO is a gaseous substance. This makes it easy to do a ligand substitution easily through purging CO out of the reaction vessel.

When considering the binding of -CO ligands to a metal center, we can see that it mostly occurs through the bond formation between the lone electron pair of CO molecule and the metal center. Here, the CO ligand acts as a terminal ligand because it has only one chemical bond on one side. The bonding of the CO ligand occurs through the Carbon atom of the CO molecule. Moreover, the sharing of this lone electron pair can occur between one, two, or even three metal centers. Here, these metal centers should be interconnected with metal-metal bonds. When more than one metal center is involved in the formation of carbonyl complexes, -CO ligand acts as a bridged ligand. Furthermore, if the two metals that are in the metal-metal bond are different from each other, that means one metal can strongly attract the CO ligand than the other metal according to electronegativity. In this type of situation, CO ligand acts as a semi-bridging ligand.

Furthermore, -CO ligand in carbonyl complexes may occur in the pi-electron donor type as well. Here, the pi electrons of the CO molecule are involved in electron donation to the metal orbitals. But these complexes can use these electrons in conjugation with the electron pair of the carbon atoms of the CO ligand. This is because the lone electron pair of the carbon atom has high energy compared to the pi electrons.

Examples of Carbonyl Complexes

Some examples for carbonyl complexes are:

[Co(CO)₆]³⁺
[Fe(CO)₆]²⁺
[Rh(CO)₆]³⁺
[Mn(CO)₆]⁺
[V(CO)₆]^–

What are Nitrosyl Complexes?

Nitrosyl complexes are coordinate complexes consisting of a metal center and nitric oxide ligands. The chemical formula of this ligand is -NO, which is bound to a transition metal center. Mostly, this ligand occurs as the nitrosyl cation: NO+. This cation is isoelectronic with CO ligand. Therefore, the bonding between a metal and NO and the bond between metal and CO ligand have the same properties.

The NO ligand acts as a two-electron donor where it shares the lone electron pair with the metal center. Furthermore, it can accept electrons from the metal through the back bonding process. However, in the context of electron-counting, two linear NO ligands are chemically equivalent to three linear CO ligands.

Figure 01: Linear and Bent Nitrosyl Ligands

Moreover, there can be linear or bent nitrosyl ligands. Linear NO ligand has three covalent bonds between N and O atoms, while bent NO ligand has two covalent bonds between N and O atoms. Generally, the NO ligand forms a linear bond between the transition metal center and the ligand, whereas bent NO ligands occur when the back bonding process is less important.

What is the Difference Between Carbonyl and Nitrosyl Complexes?

Carbonyl complex and nitrosyl complex are two types of coordination complexes containing a transition metal center and ligands surrounding the metal center. The key difference between carbonyl and nitrosyl complexes is that carbonyl complexes contain -CO ligands, whereas nitrosyl complexes contain -NO ligands.

The below infographic lists the differences between carbonyl and nitrosyl complexes in tabular form.

Summary – Carbonyl vs Nitrosyl Complexes

Carbonyl complexes are coordinate complexes consisting of a metal center and carbonyl ligands. Nitrosyl complexes are coordinate complexes consisting of a metal center and nitric oxide ligands. Therefore, the key difference between carbonyl and nitrosyl complexes is that carbonyl complexes contain -CO ligands, whereas nitrosyl complexes contain -NO ligands.