This perspective discusses the principles of the multistate scenario often encountered in transition metal catalyzed reactions, and is organized as follows. First, several important theoretical concepts (physical versus formal oxidation states, orbital interactions, use of (spin) natural and corresponding orbitals, exchange enhanced reactivity and the connection between valence bond and molecular orbital based electronic structure analysis) are presented. These concepts are then used to analyze the electronic structure changes occurring in the reaction of C–H bond oxidation by Fe^IVoxo species. The analysis reveals that the energy separation and the overlap between the electron donating orbitals and electron accepting orbitals of the Fe^IVoxo complexes dictate the reaction stereochemistry, and that the manner in which the exchange interaction changes depends on the identity of these orbitals. The electronic reorganization of the Fe^IVoxo species during the reaction is thoroughly analyzed and it is shown that the Fe^IVoxo reactant develops oxyl radical character, which interacts effectively with the σ_CH orbital of the alkane. The factors that determine the energy barrier for the reaction are discussed in terms of molecular orbital and valence bond concepts.

Iron porphyrins can act as potent electrocatalysts for CO₂ functionalization. The catalytically active species has been proposed to be a formal Fe(0) porphyrin complex, [Fe(TPP)]^2– (TPP = tetraphenylporphyrin), generated by two-electron reduction of [Fe^II(TPP)]. Our combined spectroscopic and computational investigations reveal that the reduction is ligand-centered and that [Fe(TPP)]^2– is best formulated as an intermediate-spin Fe(II) center that is antiferromagnetically coupled to a porphyrin diradical anion, yielding an overall singlet ground state. As such, [Fe(TPP)]^2– contains two readily accessible electrons, setting the stage for CO₂ reduction.

This perspective revisits Charles Coulson’s famous statement from 1959 - “give us insight not numbers” in which he pointed out that accurate computations and chemical understanding often do not go hand in hand. We argue that today, accurate wavefunction based first-principle calculations can be performed on large molecular systems, while tools are available to interpret the results of these calculations in chemical language. This leads us to modify Coulson’s statement to “give us insight and numbers”. Examples from organic, inorganic, organometallic and surface chemistry as well as molecular magnetism illustrate the points made.