Iron porphyrin carbenes (IPCs) are important reaction intermediates in engineered carbene transferase enzymes and homogeneous catalysis. However, discrepancies between theory and experiment complicate the understanding of IPC electronic structure (i.e., open- vs. closed-shell singlet (OSS vs. CSS)). Here we investigate the structurally dependent ground and excited spin state energetics of a free carbene and its IPC analogs. Only multireference <i>ab initio</i> wave function methods are consistent with experiment and predict a CSS ground state (Fe(II)←{:C(X)Y}<sup>0</sup>), contrary to density functional theory (DFT). The OSS is a high-lying metal-to-ligand charge transfer (MLCT) excited state that is sensitive to the nature of the axial ligand. Furthermore, potential energy surfaces (PESs) along the Fe–C bond elongation coordinate exhibit strong mixings between CSS/OSS characters, which can be an important feature for describing reaction mechanisms. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character. <br>
Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes
