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Treatment of Cp*2Os2Br4 with 3-bromo-2-methylpropene or 1,3-cyclooctadiene affords the 2-methylpropenyl and cyclooctenyl products Cp*Os(η³-C4H7)Br2 and Cp*Os(η³-C8H13)Br2, respectively. The monoalkyl complexes Cp*Os(η³-C4H7)MeBr and Cp*Os(η³-C4H7)(CH2SiMe3)Br can be generated by treating Cp*Os(η³-C4H7)Br2 with MgMe2 or LiCH2SiMe3. The dimethyl complex Cp*Os(η³-C4H7)Me2 can be synthesized from Cp*Os(η³-C4H7)Br2 by addition of methyllithium. In contrast, addition of ethyllithium to Cp*Os(η³-C4H7)Br2 yields the ethylene complex Cp*Os(η³-C4H7)(η²-C2H4) and a small amount of Cp*Os(η³-C4H7)H2. The latter dihydride can be synthesized in better yield by treating Cp*Os(η³-C4H7)Br2 with LiAlH4. When the dimethyl complex Cp*Os(η³-C4H7)Me2 is protonated with HBF4 in the presence of H2O, the aqua complex [Cp*Os(η³-C4H7)Me(H2O)][BF4] is formed.

The preparations of several new (pentamethylcyclopentadienyl)osmium(II) complexes from the osmium(III) compound (C5Me5)2Os2Br4 are described; among these are phosphine and alkene complexes of stoichiometry (C5Me5)OsL2Br and (C5Me5)OsL2H as well as the nitrosyl complex (C5Me5)Os(NO)Br2. Treatment of (C5Me5)2Os2Br4 with PPh3 in ethanol or PMe3 in dichloromethane affords the osmium(II) complexes (C5Me5)OsL2Br, where L = PPh3 or PMe3; the 1,5-cyclooctadiene complex (C5Me5)Os(cod)Br can be made similarly in ethanol. Treatment of either the PPh3 or cod complex with other tertiary phosphines in refluxing heptane affords several other compounds of this class:  (C5Me5)OsL2Br, where L = PEt3, ¹/2 Me2PCH2PMe2, ¹/2 Me2PCH2CH2PMe2, or ¹/2 Ph2PCH2PPh2. These bromoosmium(II) species serve as excellent starting materials for the preparation of other osmium(II) complexes. For example, treatment with NaBH4 in ethanol or with NaOMe in methanol affords the hydrides (C5Me5)OsL2H, where L = PMe3, PEt3, PPh3, ¹/2 cod, ¹/2 Me2PCH2PMe2, ¹/2 Me2PCH2CH2PMe2, or ¹/2 Ph2PCH2PPh2. Interestingly, treatment of (C5Me5)Os(PMe3)2Br with NaBH4 in refluxing ethanol affords the dihydride cation [(C5Me5)Os(PMe3)2H2⁺], which can be deprotonated with methyllithium in tetrahydrofuran to afford the electrically neutral hydride (C5Me5)Os(PMe3)2H. This hydride complex is expected to be one of the most basic transition metal complexes known. Finally, treatment of (C5Me5)2Os2Br4 with nitric oxide in dichloromethane yields the osmium(II) complex (C5Me5)Os(NO)Br2. IR, NMR, and mass spectra of the new complexes are described. A secondary ¹³C/¹²C isotope effect on the ³¹P NMR chemical shifts of ca. 0.025 ppm is noted in several compounds. Comparisons of these osmium(II) compounds with analogous ruthenium species suggests that the former have stronger metal−ligand bonds, are slower to undergo nucleophilic substitution reactions, and are stronger reducing agents.

Sulfur mustard is an alkylating agent that causes severe, potentially debilitating blisters following cutaneous exposure. Its mechanism of pathogenesis is unknown and no antidote exists. The biochemical basis of sulfur mustard's vesicating activity has been hypothesized to be a cascade of events beginning with alkylation of DNA.Using human cells in culture, we have assessed the effects of sulfur mustard on cell cycle activity using flow cytometry with propidium iodide. Two distinct patterns emerged: a G1/S interface block at concentrations equivalent to vesicating doses (> 50 μM) and a G2 block at 10-fold lower concentrations. In addition, noticeable increases in amount of dye uptake were observed at 4 and 24 hr after sulfur mustard exposure. These increases are believed to be related to DNA repair activities and can be prevented by treatment of the cells with niacinamide, which inhibits DNA repair. Other drugs that provide alternative alkylating sites or inhibit cell cycle progression were shown to lower the cytotoxicity of sulfur mustard and to protect against its direct DNA-damaging effects.