A linear cobalt(II) complex with maximal orbital angular momentum from a non-Aufbau ground state

Orbital angular momentum is a prerequisite for magnetic anisotropy, although in transition metal complexes it is typically quenched by the ligand field. By reducing the basicity of the carbon donor atoms in a pair of alkyl ligands, we synthesized a cobalt(II) dialkyl complex, Co(C(SiMe₂ONaph)₃)₂ (where Me is methyl and Naph is a naphthyl group), wherein the ligand field is sufficiently weak that interelectron repulsion and spin-orbit coupling play a dominant role in determining the electronic ground state. Assignment of a non-Aufbau (d _{<i>x</i> ²} _{<i>-y</i> ²} , d <i>_xy</i> )³(d <i>_xz</i> , d <i>_yz</i> )³(d _{<i>z</i> ²} )¹ electron configuration is supported by dc magnetic susceptibility data, experimental charge density maps, and ab initio calculations. Variable-field far-infrared spectroscopy and ac magnetic susceptibility measurements further reveal slow magnetic relaxation via a 450-wave number magnetic excited state.