Oxidation states "naturally" : A Natural Bond Orbital method for determining transition metal oxidation states

The oxidation state (OS) concept is arguably one of the most useful formalisms in chemistry. OSs are used to explain a variety of phenomena at transition metal centers, from chemical reactivity to spectroscopic properties. Attempting to define a theoretical method of evaluating this construct, however, has resulted in a broad debate among chemists, particularly inorganic chemists. With this in mind we propose a simple method for determining the oxidation states of transition metal centers using Natural Bond Orbital (NBO) theory. A description of the wavefunction (or electron density in the case of density functional theory, as presented in this investigation) is obtained from quantum chemical calculations. The 5 × 5 d-orbital Natural Atomic Orbital (NAO) occupation matrix is then obtained, and diagonalized. The resulting eigenvalues deliver the d-orbital occupations, from which the oxidation states can be inferred. The NBO-driven method also allowed us to probe "ambiguous" cases where a strong π-acid is involved in bonding (in our case, CO). The scope of the method is described, along with promising future applications.