Ligand effects on the structures of Rh-6(CO)(15)L clusters

The crystal and molecular structures of the hexarhodium carbonyl clusters R h-6(CO)(15)L (L = P(OPh)(3), 1; P(4-XC6H4)(3), (X = CF3, 2; Cl, 3; F, 4; OM e, 5); P(Bu-n)(3), 6; Me2SO, 7; MeCN, 8; C8H14, 9) have been determined by single crystal X-ray crystallography. Redetermination of the structure of t he parent Rh-6(CO)(16) cluster, 10, has also been performed and all the res ults are compared with those for the rather disparate group of monosubstitu ted 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 dist ortions with a high degree of stereoselectivity in the vicinity of the subs titution site, and demonstrate the significantly local character of the eff ect. The effects on the terminal CO groups are mainly limited to the CO att ached 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 pr ojecting above that triangle, shows a pronounced lengthening of the Rh-Rh b onds 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 ef fect is the displacement of the CO groups in triply bridged Rh-3 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 as sociated with an increase of electron density on the substituted Rh atom. T he other monosubstituted clusters that contain quite different neutral and anionic heteroligands do not display common trends in the structural distor tions induced by the substituents, and this is evidently due to the very va ried nature of the ligands within this group.