Reaction of 1,2,3,4-tetraphenyl-1,2,3,4-tetraphospholane (1) with [Os-3(CO)(11)(NCMe)] at ambient temperature affords substituted clusters: the monosubstituted trinuclear cluster [Os-3(CO)(11){(PPh)(4)CH2}] (1) and the isomeric linked bis-trinuclear clusters [{Os-3(CO)(11)}(2){mu-1,4-eta(2)-(PPh)(4)CH2}](2) and [{Os-3(CO)(11)}(2){mu-1,3-eta(2)-(PPh)(4)CH2}] (3). Clusters 2 and 3 can also be prepared by further reaction of 1 with [Os-3(CO)(11)(NCMe)]. The reaction at 100 degreesC gives, apart from cluster 2, the disubstituted 1,4-bridged trinuclear cluster [Os-3(CO)(10){mu-1,4.eta(2)-(PPh)(4)CH2}] (4). The conversion of 1 into 4 can be achieved through the pyrolysis of a solution of 1. When 1 reacts with an equimolar amount of [Os-3(CO)(10)(mu-H)(2)] at 100 degreesC in toluene, the 1,2,4-linked bis-trinuclear cluster [Os-3(CO)(11){mu(3)-1,2,4-eta(3)-(PPh)(4)CH2}-Os-3(CO)(8)(mu-H)(2)] (5) is obtained. When I reacts with a 2-fold molar amount Of [Os-3(CO)(10)(mu-H)(2)] the 1,2,3,4-linked bis-trinuclear hydride cluster [{Os-3(CO)(8)(mu-H)(2)}(2){mu(4)-1,2,3,4-eta(4)-(PPh)(4)CH2}] (6) is obtained. Cluster 1 exists as two conformational isomers (1y and 1r) in the crystalline state, due to different conformational arrangements of pseudoaxial carbonyls in the cluster. Cluster 3 shows two interconvertible conformers (3y and 3r) due to the inversion of the configuration of the uncoordinated outer phosphorus atom, and a pair of enantiomers exists in 3r. All of the new compounds obtained have been characterized by spectroscopic and analytical techniques, and their structures have been established by X-ray crystallography.