The reactions of the osmium-tin cluster complexes Os-3(CO)(12)(mu(3)-SnPh)Ph, 9, and Os-4(CO)(16)(mu(4)-Sn), 10, with Pt(PBu3t)(2) have been investigated. Two products, PtOs3(CO)(12)(Ph)(PBu3t)(mu(3)-SnPh), 11, and Pt2Os3(CO)(12)(mu(2)-Ph)(PBu3t)(2)(mu(3)-SnPh), 12, were obtained from the reaction of 9 with Pt(PBu3t)(2). These are mono- and bis-Pt(PBu3t) adducts of 9 formed by the addition of a Pt(PBu3t) group to the Os-Os bond in 11 and the Os-Os bond and Os-C bond to the sigma-bonded phenyl group in 12. A PBu3t derivative of 11, Os-2(CO)(8)(mu(3)-SnPh)Os(CO)(3)(PBu3t)Ph, 13, was obtained by treating 12 with PBu3t. The reaction of 10 with Pt(PBu3t)(2) provided the bis-Pt(PBu3t) adduct Os-4(CO)(16)[Pt(PBu3t)](2)(mu(4)-Sn), 14, that was formed by the addition of a Pt(PBu3t) group across the Os-Os bond of both Os-2(CO)(8) groups in 10. All four new compounds 11-14 were characterized by single-crystal X-ray diffraction analysis.