Mixed metal cluster chemistry. 10. Isomer distribution and ligand fluxionality at CpWIr3(mu-CO)(3)(CO)(8-n)(PR3)(n) (n=1, 2; R = Ph, Me)

The impact upon ligand coordination geometry and carbonyl fluxionality of h eterometal 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-containin g Ir(CO)(3) vertex by a mid transition metal CpW(CO)(2) unit. This "very mi xed" metal cluster has been derivatized by phosphines, the ligand fluxional ity of the resultant clusters has been examined, and both the coordination geometry and CO mobility of the tungsten-iridium clusters have been contras ted with those of derivatives of the "parent" homometallic cluster. The tet rahedral 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 interconv erting isomers in solution. The structures of the isomers have been assigne d using a combination of variable-temperature P-31 and C-13 NMR, COSY spect ra and X-ray structural studies. All phosphine-containing clusters contain a carbonyl-bridged basal plane and an apical metal; ligands can be approxim ately 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 configurat ion (2c; axial phosphine, radial or axial Cp, WIr2 basal plane) is observed only with the larger phosphine. The configurations of 3b and 5b (radial ph osphine, axial phosphine, radial or axial Cp, WIr2 basal plane) are consist ent with a structural determination of 3b, while a further configuration (3 a and 5a; diradial phosphines, radial Cp, WIr2 basal plane) possesses a tri radial coordination geometry. The configurations of 2a, 3a, 4a, and 5a are without precedent in monodentate ligand-substituted derivatives of the pare nt tetrairidium system. In contrast to reports with rhodium-iridium cluster s, 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 fl uxionality in the abundant isomers of 2-5, assigned using C-13 exchange spe ctroscopy (EXSY) spectra, are proposed. Clusters 2a and 4a, with mutually t rans 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 process es: scrambling about the Ira face to afford an intermediate with the carbon yl 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 intermedi ate in which the putative merry-go-round process is blocked by phosphines a nd Cp.