The First Structurally Confirmed Paddlewheel Compound with an M₂⁷⁺ Core: [Os₂(hpp)₄Cl₂](PF₆)

Oxidation of Os2(hpp)4Cl2, 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)4Cl2]PF6, 2. This is the first structurally confirmed metal−metal bonded paddlewheel compound having an M27+ core. The Os−Os distances for two crystalline forms, 2·2acetone and 2·hexane, are 2.3309(4) and 2.3290(6) Å, respectively. EPR, 1H 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.