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

Abstract 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.