The extraction of nickel using the Mond process involves the formation of a stable volatile complex. In this process, carbon monoxide gas is reacted with nickel oxide at high temperatures to produce nickel tetracarbonyl, a volatile compound. The nickel tetracarbonyl is then decomposed to obtain pure nickel.


During the Mond process, nickel tetracarbonyl (Ni(CO)4) is formed as a stable volatile complex. This complex consists of a nickel atom surrounded by four carbon monoxide (CO) ligands.


The coordination number of nickel in the Ni(CO)4 complex is 4. This means that the nickel atom is coordinated with four ligands, in this case, carbon monoxide molecules. The carbon monoxide ligands donate a lone pair of electrons from the oxygen atom to the vacant d-orbitals of the nickel atom, forming coordinate bonds.

The stability of the Ni(CO)4 complex can be attributed to several factors:
- Strong pi-bonding: The carbon monoxide ligands form strong pi-bonds with the nickel atom, resulting in a stable complex. The pi-bonding occurs between the filled pi-orbitals of the carbon monoxide ligands and the empty d-orbitals of the nickel atom.
- Back-donation: The nickel atom donates electron density back to the carbon monoxide ligands, strengthening the pi-bonds and stabilizing the complex further.
- Octet rule: The coordination of four ligands allows the nickel atom to achieve an octet configuration, which is energetically favorable.

The stability of the Ni(CO)4 complex ensures that it remains in the gaseous state at the reaction temperatures used in the Mond process. This allows for the separation of nickel tetracarbonyl from other impurities. The complex is then decomposed at a higher temperature, resulting in the release of pure nickel metal.


In the extraction of nickel by the Mond process, a stable volatile complex, nickel tetracarbonyl (Ni(CO)4), is formed. This complex consists of a nickel atom coordinated with four carbon monoxide ligands. The stability of the complex is due to strong pi-bonding, back-donation, and the attainment of an octet configuration.