We report a novel synthesis of a radical-bridged dinuclear cobalt(II) complex, with ample yield to enable in-depth physical investigations. We have analyzed the energy spectrum of mononuclear (1) and radical-bridged, dinuclear (2) four-coordinate cobalt(II) complexes in detail, by means of a combination of different spectroscopies (IR, Raman) and ab initio theory. Projection of ab initio energies on suitable Hamiltonians allows for direct comparison between experiment and theory. This investigation has revealed the following important points: First of all, the extraordinarily large zero-field splitting of these complexes is due to the presence of a low-lying electronic state with quartet multiplicity on the cobalt(II) ion. This excited state differs from the electronic ground state by the position of the electron hole in the d xy or d x 2 -y 2 orbitals. Both states mix very strongly in first order by spin-orbit coupling. Although it had been reported before that elongation along the pseudo-S 4 -axis of the nominal coordination tetrahedron engenders large ZFS, this physical insight