Physical Origin for the Unusual g -Values Observed for a Pb(I) Radical Complex: Quenching the Spin Angular Momentum Caused by Strong Spin–Orbit Coupling

Heavy main-group radical complexes likely exhibit unusual magnetic properties thanks to the intrinsically strong spin–orbit coupling (SOC) of the heavy elements. Herein, we elucidate the physical origin for the distinct magnetic anisotropy observed for group 14 Ge I, Sn I, and Pb I radical anions 1-E (E = Ge, Sn, Pb) whose doublet ground states feature an (ns) 2 (np z ) 1 electron configuration. Complexes 1-Ge and 1-Sn display moderate magnetic anisotropy manifested by g x,y,z = 1.976, 1.976, 1.999 and g x,y,z = 1.879, 1.906, 1.982, respectively, ( Angew. Chem., Int. Ed. 2022, 61, e202201248), with all g -factors close to the spin-only value, g e . In contrast, 1-Pb is distinguished by a highly anisotropic g -tensor with g x,y,z = 1.335, 1.410, and 1.693, in particular, having an exceptionally low g z -factor. Effective Hamiltonian analyses based on wave function-based ab initio calculations reveal that 1-Ge and 1-Sn possess orbitally nondegenerate ground states, wherein the weak SOC between the ground state and the two low-lying excited states with dominant electron configurations of (ns) 2 (np y ) 1 and (ns) 2 (np x ) 1 partially restores the orbital angular momentum in the x- and y -directions, resulting in g x ≈ g y < g z ≈ g e . However, the ground level of 1-Pb features a triple orbital near-degeneracy, and the ensuing strong SOC with the same excited states not only introduces the considerable orbital angular momentum in the x - and y -directions but also, more critically, substantially quenches the spin angular momentum in the z -direction due to the conservation of the total angular momentum. As a consequence, all three g -components of 1-Pb are significantly lower than g e, exhibiting a pattern of g x ≈ g y < g z < g e . This work, therefore, provides a general framework for correlating g -matrices measured experimentally with electronic structures of S = 1/2 main-group and transition-metal complexes.