Oxidation of OS2(hPP)(4)Cl-2, 1 (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), with (FeCP2)PF6 produces air-stable [OS2(hPP)(4)Cl-2]PF6, 2. This is the first structurally confirmed metal-metal bonded paddlewheel compound having an M-2(7+) core. The Os-Os distances for two crystalline forms, 2-2acetone and 2-hexane, are 2.3309(4) and 2.3290(6) Angstrom, respectively. EPR, H-1 NMR, and magnetization data indicate that 2 has an unpaired electron and an exceptionally low g value of 0.791 +/- 0.037. An electrochemical study shows that there is a quasireversible wave corresponding to a more highly oxidized species with an unprecedented OS28+ core.