The cluster [H 2Os 4(CO) 13] may be reduced with K/Ph 2CO to generate an anion which reacts with [M(η 6-C 6H 5R)(MeCN) 3] 2+ (M=Os, Ru; R=H, Me) affording the capped clusters [H 2Os 4M(CO) 12(η 6-C 6H 5R)] (M=Os, R=H ( 1); M=Ru, R=H ( 2); M=Os, R=Me ( 3)) with the arene ligand on one of the axial site of a trigonal bipyramid. The Os clusters 1 and 3 transform into the more stable equatorial isomers, 4 and 5, respectively, in 24 h. The structure of 5 has been confirmed by a single crystal X-ray diffraction study, and all the complexes have been characterised spectroscopically. The axial isomers may be carbonylated or hydrogenated to generate the edge-bridged tetrahedral clusters [H 2Os 4M(CO) 13(η 6-C 6H 5R)] (M=Os, R=H ( 6); M=Ru, R=H ( 7); M=Os, R=Me ( 8)) and the known clusters [H 4Os 4M(CO) 12(η 6-C 6H 6)] (M=Os ( 9); Ru ( 10)). The structure of 7 has been confirmed by an X-ray diffraction analysis. The axial isomer 1 reacts with PPh 2Me to yield the addition product [H 2Os 5(CO) 12(η 6-C 6H 6)PPh 2Me] ( 11), whereas the mixed osmium–ruthenium analogue undergoes a transformation to the ‘spiked’ tetrahedral [H 3Os 4Ru(CO) 12(μ 3-σ,σ,η 6-C 6H 5)PPh 2Me] ( 12). Deprotonation of the axial isomers 1 and 2 and subsequent reaction with [AuPPh 3] + yields [HOs 4M(CO) 12(C 6H 6)(AuPPh 3)] (M=Os ( 13); Ru ( 14)). As well as spectroscopic characterisation cluster, the structures of 11, 12 and 13 have been established by X-ray diffraction experiments. In a related series of reactions the cluster monoanion [Os 4(CO) 13X] − (X=Cl ( 15), Br ( 16), I ( 17)) may be capped using the monocation [Ru(η 5-C 5H 5)(MeCN) 3] + affording [HOs 4Ru(CO) 13(η 5-C 5H 5)] ( 18), which is readily deprotonated and further capped with [AuPPh 3] + to afford the neutral cluster [Os 4(CO) 13Ru(η 5-C 5H 5)AuPPh 3] ( 20). The structures of 15, 18 and 20 have been established crystallographically. The cluster anion [Os 4(CO) 13X] − may be reduced with K/Ph 2CO and the resulting anionic complex coupled with [Ru(η 5-C 5H 5)(MeCN) 3] +, providing another route to [Os 4Ru(CO) 13(η 5-C 5H 5)] − ( 19). This anionic complex may also be coupled with [Ru(η 6-C 6H 6)(MeCN) 3] 2+ affording [Os 4Ru(CO) 13(η 6-C 6H 6)] ( 21) which has also been structurally characterised.