Treatment of RHgCl (R = Me, Et, Ph, Fc) with the activated cluster [Os-3(CO)(10)(NCMe)(2)] produces the osmium-mercury mixed-metal clusters [{Os-3(CO)(10)(mu Cl)}(2)(mu(4)-Hg)] (1a) and [{Os-3(CO)(10)(mu-Cl)}{mu-HgOs(CO)(4)}(2)] (2a) in low yields. Cluster 1a bears a central wingtip mercury atom simultaneously bridging two {Os-3(CO)(10)(mu-Cl)} units, Cluster 2a comprises an unprecedented decanuclear osmium-mercury framework in which there is a central linear "Hg-{Os(CO)(4)}(2)-Hg" molecular backbone with the two Os(CO)(4) fragments arranged in a staggered conformation. Cluster 2a is thermally unstable and converts slowly to cis-[Os(CO)(4){(mu-Hg)Os-3(CO)(10)(mu-Cl)}(2)] (4a) via the extrusion of an Os(CO)4 moiety under ambient conditions, Cluster 4a comprises two {(mu-Hg)Os-3(CO)(10)(mu-Cl)} subcluster units bonded to a central Os(CO)(4) moiety in a cis configuration. Similar reactions with PhHgBr and MeHgI have also been investigated. Cluster 1a crystallizes in the triclinic space group with a = 9.112(2) Angstrom, b = 12.314(2) Angstrom, c = 15,501(4) Angstrom, alpha = 84.92(2),degrees beta = 88.26(3),degrees gamma = 79.52(2)degrees and Z = 2. Cluster 2a crystallizes in the triclinic space group with a = 13.306(2) Angstrom, b = 20.768(5) Angstrom, c = 9.771(4) Angstrom, a = 96.36(3),degrees beta = 90.50(2),degrees gamma = 98.81(2)degrees and Z = 2, ouster 4a crystallizes in the triclinic space group with a = 18.356(2) Angstrom, b = 19.100(3) Angstrom, c = 14.478(2) Angstrom, alpha = 89.783(2),degrees beta = 113.19(2),degrees gamma = 113.70(2)degrees and Z = 4.