The dn Number in Transition Metal Chemistry: Its Utility and Limitations

The dn number or configuration is a very useful parameter in transition metal chemistry because it conveys information about the expected properties of a d-block metal complex, including its shape, magnetism, thermodynamic stability, kinetic lability, and spectroscopic properties. The dn number can be determined from either the oxidation state (OS) or valence number (VN) of the metal centre, and since, in most cases, these are numerically equal, the derived dn value is the same. However, examples are discussed where the OS and VN are not equal, which results in unavoidable ambiguities in dn values. Following a discussion of these examples, a revised definition of dn is proposed based on the occupation of the frontier molecular orbitals of the complex.