The
tetrahedral cobalt(II) compound (Ph<sub>4</sub>P)<sub>2</sub>[Co(SPh)<sub>4</sub>] was the first mononuclear transition-metal complex shown
to exhibit slow relaxation of the magnetization in zero external magnetic
field. Because the relative populations of the d orbitals play a vital
role in dictating the magnitude of the magnetic anisotropy, the magnetic
behavior of this complex is directly related to its electronic structure,
yet the exact role of the soft, thiophenolate ligands in influencing
the d-electron configuration has previously only been investigated
via theoretical methods. To provide detailed experimental insight
into the effect of this ligand field, the electron density distribution
in this compound was determined from low-temperature, single-crystal
X-ray diffraction data and subsequent multipole modeling. Topological
analysis of the electron density indicates significant covalent contributions
to the cobalt–sulfur bonds. The derived d-orbital populations
further reveal a fully occupied d<sub><i>z</i><sup>2</sup></sub> orbital, minor d<sub><i>xz</i></sub> orbital population,
and nearly equal population of the d<sub><i>xy</i></sub>, d<sub><i>x</i><sup>2</sup></sub><sub>–<i>y</i><sup>2</sup></sub>, and d<sub><i>yz</i></sub> orbitals. Notably, we find that an electrostatic interaction between
Co(II) and one hydrogen atom from a thiophenolate group in the <i>xz</i> plane increases the energy of the d<sub><i>x</i><sup>2</sup></sub><sub>–<i>y</i><sup>2</sup></sub> orbital, leading to the nearly equal population with d<sub><i>xy</i></sub> and strong magnetic anisotropy