Stabilizing Coordinated Radicals via Metal–Ligand Covalency: A Structural, Spectroscopic, and Theoretical Investigation of Group 9 Tris(dithiolene) Complexes

Proper assignment of redox loci in
coordination complexes with redox-active ligands to either the metal
or the ligand is essential for rationalization of their chemical reactivity.
However, the high covalency endemic to complexes of late, third-row
transition metals complicates such assignments. Herein, we systematically
explore the redox behavior of a series of group 9 tris­(dithiolene)
complexes, [M­(mnt)3]3– (M = Ir, Rh, Co;
mnt = maleonitriledithiolate). The Ir species described comprise the
first examples of homoleptic Ir dithiolene complexes. The enhanced
metal–ligand covalency of the Ir–S interaction leads
to remarkable reactivity of [Ir­(mnt)3]3– and stabilization of mononuclear [Ir­(mnt)3]2– complex ions as well as dimerized versions featuring weak, covalent,
intermolecular S–S bonds. The dianionic Rh and Co analogues
are, in contrast, highly unstable, resulting in the rapid formation
of [Rh2(mnt)5]4– and [Co­(mnt)2]22–, respectively. The synthesized complexes were studied by single-crystal
X-ray diffraction, X-ray absorption spectroscopy, optical spectroscopy,
magnetometry, density functional theory, and spectroscopy-oriented
configuration interaction calculations. Spectroscopic and theoretical
analyses suggest that the stability of [Ir­(mnt)3]2– may be attributed to dilution of ligand radical character by a high
degree of Ir 5d character in the singly occupied molecular orbital.