We present the first direct measurement of Co-N(equatorial) coupling parameters, obtained from electron spin echo envelope modulation (ESEEM) spectroscopy, of trans-bis (dimethylglyoximato)bis(methanol)- and -bis(pyridine)cobalt(II) species. ESEEM spectroscopy provides a means to detect changes in unpaired spin density in the equatorial plane. Simulation of Fourier transformed ESEEM spectra of the low-spin cobalt(II) dimethylglyoxime complexes, collected in frozen methanol/toluene solution, reveal coupling parameters from four nearly magnetically equivalent; equatorially coordinated N-14 nuclei. Modulation of Co-N(equatorial) coupling is observed when the sigma-bonding strength of the axial ligands is increased from weakly coordinated methanol solvent to the more strongly coordinated bis(pyridine) complex. Nuclear hyperfine and quadrupole coupling parameters for the bis(methanol) and bis(pyridine) species are as follows : A(iso) = 2.1 +/- 0.1 MHz, e(2)qQ = 3.40 +/- 0.05 MHz; and A(iso) = 1.17 +/- 0.05 MHz, e(2)qQ = 3.30 +/- 0.05 MHz, respectively. Reduction in the magnitude of Co-N(equatorial) coupling for the dimethylglyoxime species is consistent with a decrease in unpaired spin density on cobalt upon coordination of pyridine. Localization of the principle axis of the nuclear quadrupole interaction within the cobalt(II) bis(dimethylglyoxime) plane (Euler angle beta = ca. 95 degrees) emphasizes the magnetic differences between dimethylglyoximes and porphine systems where the value for beta is consistently reported near 45 degrees (Magliozzo, R. S.;Peisach, J. Biochemistry, 1993, 33, 8446).