High Spin Co(I): High-Frequency
and -Field EPR Spectroscopy
of CoX(PPh<sub>3</sub>)<sub>3</sub> (X = Cl, Br)
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Abstract
The previously reported pseudotetrahedral Co(I) complexes,
CoX(PR<sub>3</sub>)<sub>3</sub>, where R = Me, Ph, and chelating analogues,
and X = Cl, Br, I exhibit a spin triplet ground state, which is uncommon
for Co(I), although expected for this geometry. Described here are
studies using electronic absorption and high-frequency and -field
electron paramagnetic resonance (HFEPR) spectroscopy on two members
of this class of complexes: CoX(PR<sub>3</sub>)<sub>3</sub>, where
R = Ph and X = Cl and Br. In both cases, well-defined spectra corresponding
to axial spin triplets were observed, with signals assignable to three
distinct triplet species, and with perfectly axial zero-field splitting
(zfs) given by the parameter <i>D</i> = +4.46, +5.52, +8.04
cm<sup>–1</sup>, respectively, for CoCl(PPh<sub>3</sub>)<sub>3</sub>. The crystal structure reported for CoCl(PPh<sub>3</sub>)<sub>3</sub> shows crystallographic 3-fold symmetry, but with three structurally
distinct molecules per unit cell. Both of these facts thus correlate
with the HFEPR data. The investigated complexes, along with a number
of structurally characterized Co(I) trisphosphine analogues, were
analyzed by quantum chemistry calculations (both density functional
theory (DFT) and unrestricted Hartree–Fock (UHF) methods).
These methods, along with ligand-field theory (LFT) analysis of CoCl(PPh<sub>3</sub>)<sub>3</sub>, give reasonable agreement with the salient
features of the electronic structure of these complexes. A spin triplet
ground state is strongly favored over a singlet state and a positive,
axial <i>D</i> value is predicted, in agreement with experiment.
Quantitative agreement between theory and experiment is less than
ideal with LFT overestimating the zfs, while DFT underestimates these
effects. Despite these shortcomings, this study demonstrates the ability
of advanced paramagnetic resonance techniques, in combination with
other experimental techniques, and with theory, to shed light on the
electronic structure of an unusual transition metal ion, paramagnetic
Co(I)