Ligand effects on the structures of Rh6(CO)15L clusters

Abstract

The crystal and molecular structures of the hexarhodium carbonyl clusters Rh₆(CO)₁₅L (L = P(OPh)₃, 1; P(4-XC₆H₄)₃, (X = CF₃, 2; Cl, 3; F, 4; OMe, 5); P(ⁿBu)₃, 6; Me₂SO, 7; MeCN, 8; C₈H₁₄, 9) have been determined by single crystal X-ray crystallography. Redetermination of the structure of the parent Rh₆(CO)₁₆ cluster, 10, has also been performed and all the results are compared with those for the rather disparate group of monosubstituted derivatives reported earlier. The structures all relate closely to that of the parent which has an octahedral array of Rh atoms with four opposite faces triply bridged by CO ligands. Each Rh atom also has two terminal CO groups attached. The substituents simply replace one of the terminal COs. A group of ligands, comprised of the P-donors and cyclooctene, all show distortions with a high degree of stereoselectivity in the vicinity of the substitution site, and demonstrate the significantly local character of the effect. The effects on the terminal CO groups are mainly limited to the CO attached to the substituted Rh atom for which there is a clear decrease in the Rh–CO distance and a smaller lengthening of the C–O distance. The triangle of Rh atoms that contains the substituted Rh atom, with the substituent projecting above that triangle, shows a pronounced lengthening of the Rh–Rh bonds cis to the heteroligand. There is an equally pronounced shortening of the Rh–Rh bond across the triangle from the substituted Rh atom. A third effect is the displacement of the CO groups in triply bridged Rh₃ triangles containing the RhL moiety, so that they almost doubly bridge the two Rh–Rh bonds cis to the substituted Rh atom. Although no clear correlations exist within this group of substituents the localized effects can generally be associated with an increase of electron density on the substituted Rh atom. The other monosubstituted clusters that contain quite different neutral and anionic heteroligands do not display common trends in the structural distortions induced by the substituents, and this is evidently due to the very varied nature of the ligands within this group.