The present work provides a useful framework through which theoreticians and practicing computational chemists alike can view electron correlations in transition metal complexes. We present static correlation from the perspective of simple chemical models such as ligand-field and molecular orbital theories, and an analysis of symmetry breaking suggests that it is rarely encountered in thermochemical calculations of realistic transition metal compounds (though we do reveal a few important situations in which it is relevant). The relative complexity of transition metal bonding, relative to that of typical organic compounds, places a larger burden on a theory's treatment of dynamic correlation. After recognizing that simultaneous sigma-donation and pi-backbonding can be viewed as a correlated interaction involving multiple electron pairs, we demonstrate that MP2 and related double hybrid density functionals fail to describe this type of bonding.
Correlation in Metal Complexes SI