The cluster cis-[Re6Se8(PEt3)(4)I-2] undergoes substitution reactions with neutral ligands L in the presence of AgSbF6 to afford the cis-disubstituted clusters [Re6Se8(PEt3)(4)L-2](2+) (L = MeCN (6), t-BuCN, DMF, Me2SO (9)). Cluster 6 reacts with pyridine to give [Re6Se8(PEt3)(4)(py)(2)](2+). Retention of stereochemistry was demonstrated from P-31 NMR spectra and the crystal structure of [9](SbF6)(2), which revealed the two Me2SO ligands to be O-bonded at adjacent Re sites of the face-capped octahedral [Re-6(mu(3)-Se)(8)](2+) core. The clusters cis-[Re6Se8(PEt3)(4)L-2](2+) are of interest as possible precursors to oligomeric clusters under thermolysis conditions where one or both ligands L are removed. Thermolysis of orange-red [6](SbF6)(2) (180 degrees C, 24 h) afforded a brown-green crystalline product identified by an X-ray structure determination to contain the dicluster [Re12Se16(PEt3)(8)(MeCN)(2)](4+) (11). Two [Re6Se8](2+) cluster units are bridged by a Re2Se2 rhomb to afford the centrosymmetric core [Re-12(mu(3)-Se)(14)(mu(4)-Se)(2)](4+), established recently for the dicluster [Re12Se16(PEt3)(10)](4+). The two acetonitrile ligands occur in a trans orientation on the two cluster units. From consideration of the crystal structure of the precursor compound [6]-(SbF6)(2) (reported earlier), the relative orientation of cluster pairs could be responsible for the formation of trans-11. Diclusters of the type cis-[Re12Se16(PEt3)(8)L-2](4+) are required for potential thermolytic conversion to the cyclic tetracluster [Re24Se32(PEt3)(16)](8+). Factors relevant to the formation of this unknown cluster type are considered.