LIGAND SUBSTITUTION PROCESSES IN TETRANUCLEAR CARBONYL CLUSTERS .6. STERIC CONTRIBUTION TO LIGAND DISSOCIATION IN MULTI-SUBSTITUTED IR4(CO)12 DERIVATIVES
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A kinetic investigation of carbon monoxide exchange processes in the series of trisubstituted Ir4(CO)9[PR3]3 derivatives (R = Me, Et, n-Bu, Ph, and i-Pr) is reported. The rate of dissociation of CO was accelerated as the spatial requirements of the substituted PR3 ligand increased, with the order of CO-labilizing ability being PMe3 < P-n-Bu3 PEt3 < P-i-Pr3 < PPh3. A smaller electronic effect of the phosphorus ligand is superimposed on this steric acceleration phenomenon as noted for the PPh3 case. The overall increase in dissociative CO rates resulting from replacement of three carbon monoxide ligands in the parent Ir4(CO)12 species by PR3 varied over a factor of 115045 100 as the cone angle of the trialkylphosphines increased from 118 to 160 C while their basicities were essentially unchanged. The activation parameters for CO dissociation in the representative Ir4(CO)9(PEt3)3 species were determined to be H* = 28.4 2.7 kcal/mol and S* = 3.52 6.42 eu, values consistent with a dissociative process. A similar kinetic lability of the phosphorus ligand in Ir4(CO)8(PR3)4 derivatives was noted for sterically demanding PR3 ligands. For Ir4(CO)8(PR3)4 derivatives possessing PR3 ligands of uniform electronic character, R = Me, Et, and n-Bu, the rate of phosphine dissociation increased by a factor of 3300 with increasing ligand cone angle. Activation parameters (H* and S*) for PEt3 dissociation in Ir(CO)8(PEt3)4 were found to be 33.0 2.7 kcal/mol and 19.1 7.9 eu, respectively. A general discussion of the origin of steric and electronic phosphorus ligand labilizing abilities in metal clusters is provided. 1982, American Chemical Society. All rights reserved.