CASSCF-based explicit ligand field models clarify the ground state electronic structures of transition metal phthalocyanines (MPc; M = Mn, Fe, Co, Ni, Cu, Zn)

Abstract

Multireference electronic structure methods are used to assign ground state electronic configurations for a series of metallophthalocyanines. Ligand orbital occupancies remain constant across the period and are consistent with a formal 2– charge on the ligand. The d electron configurations of some metallophthalocyanines are straightforward and can be unambiguously assigned, (dxy)2(dxz,dyz)2,2( [Formula: see text])2([Formula: see text])n, with n = 2, 1, 0, respectively, for ZnPc, CuPc, and NiPc. Controversies over ground state electronic structure assignments for other metallophthalocyanines arise due to multiple complicating factors: accidental near-degeneracies, environmental effects, and different ligand field models used in interpreting experimental spectra. We demonstrate that explicit ligand field models provide more reliable and consistent interpretations of experimental data than implicit, parameterized alternatives. On this basis, we assign gas-phase electronic ground states for MnPc, (dxy)2(dxz,dyz)1,1([Formula: see text])1 and CoPc, (dxy)2(dxz,dyz)2,2([Formula: see text])1, and show that the ground state of FePc cannot be resolved to a single state, with two near-degenerate states that are likely spin-orbit coupled: (dxy)2(dxz,dyz)1,1( [Formula: see text])2 and (dxy)2(dxz,dyz)2,1([Formula: see text])1. Remaining differences between computational predictions and experimental observations are small and may be ascribed primarily to environmental effects but are also partly due to incomplete modelling of electron correlation.