The impact upon ligand coordination geometry and carbonyl fluxionality of heterometal incorporation into the prototypical tetrahedral cluster Ir-4(CO)(12) has been assessed. The isostructural CpWIr3(CO)(11) (1) is related to Ir-4(CO)(12) by conceptual replacement of a late transition metal-containing Ir(CO)(3) vertex by a mid transition metal CpW(CO)(2) unit. This "very mixed" metal cluster has been derivatized by phosphines, the ligand fluxionality of the resultant clusters has been examined, and both the coordination geometry and CO mobility of the tungsten-iridium clusters have been contrasted with those of derivatives of the "parent" homometallic cluster. The tetrahedral clusters CpWIr3(mu-CO)(3)(CO)(8-n)(L)(n) [L = PPh3, n = 1 (2), 2 (3); L = PMe3, n = 1 (4), 2 (5)] are shown to exist as mixtures of interconverting isomers in solution. The structures of the isomers have been assigned using a combination of variable-temperature P-31 and C-13 NMR, COSY spectra and X-ray structural studies. All phosphine-containing clusters contain a carbonyl-bridged basal plane and an apical metal; ligands can be approximately coplanar (radial) to the basal plane or below the plane (axial). The configurations of 2a and 4a (axial phosphine, apical Cp, Ir-3 basal plane) and 2b and 4b (radial phosphine, radial or axial Cp, WIr2 basal plane) are consistent; with structural determinations of 4a and 2b; a third configuration (2c; axial phosphine, radial or axial Cp, WIr2 basal plane) is observed only with the larger phosphine. The configurations of 3b and 5b (radial phosphine, axial phosphine, radial or axial Cp, WIr2 basal plane) are consistent with a structural determination of 3b, while a further configuration (3a and 5a; diradial phosphines, radial Cp, WIr2 basal plane) possesses a triradial coordination geometry. The configurations of 2a, 3a, 4a, and 5a are without precedent in monodentate ligand-substituted derivatives of the parent tetrairidium system. In contrast to reports with rhodium-iridium clusters, we find that the presence of the more electropositive tungsten does not polarize the electron distribution toward the iridiums and thereby favor an Ir-3 basal plane; rather, the WIr2-bridged form is predominant across the isomers in the tungsten-iridium system. Suggested mechanisms of carbonyl fluxionality in the abundant isomers of 2-5, assigned using C-13 exchange spectroscopy (EXSY) spectra, are proposed. Clusters 2a and 4a, with mutually trans phosphine and Cp, exchange carbonyls via a merry-go-round at Ir-3 and WIr2 faces. Clusters 3a and 5a exchange all carbonyls by way of two processes: scrambling about the Ira face to afford an intermediate with the carbonyl distribution of the D-2d form implicated in carbonyl mobility at Ir-4(CO)(12), and scrambling about a WIr2 face by way of an all-terminal intermediate in which the putative merry-go-round process is blocked by phosphines and Cp.