Ab Initio Methods in First-Row Transition Metal Chemistry

Molecules containing 3d transition metals (TMs) are usually associated with versatile reactivity, partly due to their complicated electronic structures involving multiple close-lying spin states. An accurate description of different electronic states is notoriously difficult and still a challenge for computational methodologies. Density functional theory, although repeatedly shown to give less reliable results for near-degeneracy problems, remains the workhorse to explore the reactivity of TM complexes. Ab initio wavefunction theory has been lagging behind due to its high computational cost. However, tremendous efforts have been put to improve their applicability over the past few years, particularly local coupled cluster and low-scaling multireference methods. In this mini-review, we highlight major advancements of these ab initio techniques and discuss their recent applications in the calculations of spin state energetics of first-row TM complexes, ranging from spin crossover compounds and metalloproteins to biomimetic complexes capable of activating C-H bonds.