Probing electronic structures of transition metal complexes using electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) has been widely employed to characterize transition metal complexes. However, because of the high degree of complexity of transition metal EPR spectra, how to extract the underlying electronic-structure information inevitably poses a major challenge to beginners, in particular for systems with $S>\frac{1}{2}$. In fact, the physical principles of transition metal EPR have long been well-established and since 1970s a series of dedicated voluminous monographs have been published already. Not surprisingly, they are not appropriate stating points for novices to grasp a panorama of the profound theory prior to scrutinizing in-depth references. The present review aims to fill this gap to provide a perspective of transition metal EPR and unveil some peculiar subtleties thereof on the basis of our recent work.