Mixed platinum-rhodium carbonyl clusters. Synthesis and chemical characterization of the [PtRh6(.mu.3-CO)3(.mu.-CO)5(CO)8]2- anion and x-ray crystal structure of its tetrapropylammonium salt

The dianion [PtRh6(C0)16]~- has been obtained through several synthetic routes, particularly by redox condensation of [PtRh,(CO),,]- and [Rh(CO)J. The product is unstable in solution, both under N2, where it slowly yields species of higher nuclearity, and under CO, where it is fragmented in smaller clusters. The [N-n-Prp]*[PtRh,(C0),6] salt has been characterized by X-ray diffraction. Crystal data: CaH~6N2016PtRh6, M, = 1633.41, monoclinic, space group C2/c (No. 15), a = 17.180 (2) A, b = 15.848 (2) A, c = 39.719 (4) A, j3 = 92.45 (3)O, Z = 8, R = 0.048, R, = 0.063. The metal skeleton of the dianion is an octahedron of Rh atoms asymmetrically capped by a Pt atom. Three carbonyls out of 16 are triply bridging, five are edge bridging, and eight are terminal (two on a Rh atom of the capped face and one on each of the other metals). One of the two terminal carbonyls bound to the unique rhodium is heavily distorted (Rh-C-0 = 158 (3)O) by a weak semibridging interaction with the Pt atom (Pt-C = 2.73 (3) A).