Reaction of WH(CO) 3 (η-C 5 Me 5 ) with IrCl(CO) 2 (4-H 2 NC 6 H 4 Me) affords WIr 3 (μ-CO) 3 (CO) 8 (η-C 5 Me 5 ) in low yield. A structural study reveals a WIr 2 -centred plane of bridging carbonyls, in contrast to the crystal structure of WIr 3 (CO) 1 1 (η-C 5 H 5 ) (all-terminal carbonyl distribution). DFT calculations reveal an increasing proclivity to adopt an all-terminal CO disposition for clusters MIr 3 (CO) 1 1 (η-C 5 H 5 ) in the gas phase on proceeding from M=Cr to Mo and then W, consistent with structural studies in the solid state for which the tungsten-containing cluster is the only all-terminal example. Increasing electron donation from the ligands in the tungsten system (either from phosphine substitution or cyclopentadienyl permethylation) suffices to impose a plane of bridging carbonyls in the ground state structure. 1 3 C NMR fluxionality studies reveal that CO exchange mechanism(s) for WIr 3 (CO) 1 1 (η-C 5 H 5 ) and the related tetrahedral cluster W 2 Ir 2 (CO) 1 0 (η-C 5 H 5 ) 2 are very fast and involve all carbonyls on the clusters. DFT calculations on MIr 3 (CO) 1 1 (η-C 5 H 5 ) (M=Cr, Mo) substantiate a 'merry-go-round' mechanism for carbonyl scrambling in these systems, a result which is consistent with the scrambling behaviour seen in the NMR fluxionality studies on the W-containing congener.