Variable temperature structural and EPR studies are reported on the paddlewheel compound [Os-2(hpp)(4)Cl-2]PF6, 1, (hpp = the anion of the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) that contains a rare M-2(7+) species, with the goal of determining whether the unpaired electron resides in a metal- or ligand-based molecular orbital. Crystallographic studies show that the Os-Os distance in 1 remains essentially unchanged from 213 to 30 K, which is consistent with no changes in electronic structure in this range of temperature. It is noteworthy that the metal-metal distance in 1 is about 0.05 angstrom shorter than that in the precursor Os-2(hPP)(4)Cl-2, which is consistent with the loss of an electron in a delta(star) orbital. EPR spectra of 1 were measured in dilute frozen solution, powder, and single crystals. The spectra were observable only below about 50 K, with an exceptionally large line width, similar to 3 750 gauss, for a powdered sample, due to dipolar interactions and to short relaxation times. There is a very small average g value of similar to 0.750 and a cylindrical symmetry about the Os-Os bond. These data are consistent with the unpaired electron orbital having a large L value, such as that of a delta(star) orbital. The combination of X-ray structural data, the short relaxation time, and the magnetic data provide strong evidence that the unpaired electron in this nine-electron Os-2(7+) species is localized in a metal-based orbital with this electron residing predominantly in a delta(star) orbital rather than in a pi(star) orbital and, thus, having an electronic configuration of sigma(2)pi(4)delta(2)delta(star).