This paper describes in a somewhat personal way an overview of the use of electron paramagnetic resonance (EPR) spectroscopy, including high-frequency and -field EPR (HFEPR) to unravel the electronic structure of transition metal ion complexes. The spin Hamiltonian parameters obtained from EPR experiments, namely the g matrix for systems with S =1/2 and the g and D (zero-field splitting) matrices for systems with S > 1/2 provide information on d orbital energy levels. This information can be combined with ligand-field theory (LFT) to provide information on the overall electronic structure of the paramagnetic transition metal complex. As has been discussed by others, LFT is still useful in providing such a quantitative understanding of these complexes, even in the day of advanced computational methods, such as density functional theory (DFT). The discussion is illustrated by examples across the d n configuration.
coordination chemistry; EPR; ligand-field theory; spectroscopy
