LAUSR

Abstract Effective strategies for designing Fe(II) coordination complexes with specifically tailored spin-state energetics can lead to advances in many areas of inorganic and materials chemistry. These include, but are not limited to, rational development of novel spin crossover complexes, efficient chromophores for photosensitization of dye-sensitized solar cells, and multifunctional materials. As the spin-state ordering of transition metal complexes is strongly rooted in their electronic structures, computational chemistry has naturally played an important role in assisting experimental work in this area. Unfortunately, despite many advances, accurate determination of the spin-state energetics of Fe(II) complexes still poses a remarkable challenge for virtually all applicable forms of electronic structure theory due to being controlled by a delicate balancing between correlation and exchange effects. This review focuses on some of the more notable successes and failures of modern electronic structure theory in properly describing these systems in the absence of solid-state effects. The strengths and weaknesses of using traditional wavefunction based methods and density functional theory are considered, and illustrative examples are provided to demonstrate that the modern computational chemist should make use of experimental data whenever possible and expect to utilize a combination of methods to obtain the best results. The review closes by briefly surveying some of the many interesting combined computational and experimental studies of Fe(II) chemistry that have lead to greater fundamental insight and practical understanding of this challenging class of systems.