Homoleptic Organocobalt(III) Compounds with Intermediate Spin

Homoleptic
organocobalt­(III) compounds with formula [NBu4]­[CoIII(C6X5)4] [X = F (3), Cl (4)] were obtained in reasonable yields
by chemical oxidation of the corresponding divalent species [NBu4]2­[CoII(C6X5)4] [X = F (1), Cl (2)]. The
[CoIII(C6X5)4]−/[CoII(C6X5)4]2– couples are electrochemically related by quasi-reversible, one-electron
exchange processes at moderate potential: E
1/2 = −0.29 (X = F) and −0.36 V (X = Cl) versus saturated
calomel electrode. The [CoIII(C6X5)4]− anions in salts 3 and 4 show an unusual square-planar geometry as established by
single-crystal X-ray diffraction methods. According to their stereochemistry,
these CoIII derivatives (d6) are paramagnetic
non-Kramers systems with a large zero-field splitting contribution
and no observable electron paramagnetic resonance (EPR) spectrum.
The thermal dependence of their magnetic susceptibilities can be explained
in terms of a spin-Hamiltonian formalism with S =
1 ground state (intermediate spin) and substantial spin–orbit
contribution. The magnetic properties of the square-planar d7 parent species [NBu4]2­[CoII(C6X5)4] were also thoroughly studied
both at microscopic (EPR) and macroscopic levels (alternating current
and direct current magnetization measurements). They behave as S = 1/2 (low spin) systems with mainly (dz2)1 electron configuration and a certain
degree of s-orbital admixture that has been quantified. The electronic
structures of all four open-shell [Co­(C6X5)4]q− compounds (q = 1, 2) accounting for their respective magnetic properties
are based on a common orbital energy-level diagram.