SYNTHETIC, STRUCTURAL, SPECTROSCOPIC, AND THEORETICAL-STUDIES OF STRUCTURAL ISOMERS OF THE CLUSTER PT-3(MU-PPH(2))(3)PH(PPH(3))(2) - A UNIQUE EXAMPLE OF CORE ISOMERISM IN PHOSPHINE PHOSPHIDO-RICH CLUSTERS

Two isomers of the phosphido-bridged platinum cluster Pt-3(mu-PPh(2))( 3)Ph(PPh(3))(2) (2 and 3) have been isolated, and their structures hav e been solved by single-crystal X-ray diffraction. Compound 2 crystall izes in the orthorhombic space group Cmc2(1) with a 22.192(10) Angstro m, b = 17.650(9) Angstrom, c 18.182(8) Angstrom, and Z = 4. Compound 3 crystallizes with 2 molecules of dichloromethane in the monoclinic sp ace group C2/c with a = 21.390(10) Angstrom, b 18.471(9) Angstrom, c = 19.021(11) Angstrom, beta = 105.27(5)degrees, and Z = 4. The two isom ers differ essentially in their metal-metal distances and Pt-(mu-PPh(2 ))-Pt angles. Thus 2, having an imposed C-s, symmetry, contains a bent chain of metal atoms with two short Pt-Pt distances of 2.758(3) Angst rom and a long separation of 3.586(2) Angstrom. In 3, which has an imp osed C-2 symmetry, the metal atoms form an isosceles triangle with two Pt-Pt distances of 2.956(3) Angstrom and one of 3.074(4) Angstrom. Th ese isomers can be smoothly interconverted by changing the crystalliza tion solvents. Solution and solid-state P-31 NMR studies have been per formed in order to assign the resonances of the different P nuclei and relate their chemical shifts with their structural environments. Rama n spectroscopy was used to assign the nu(Pt-Pt) modes of the two struc tural isomers. Theoretical studies based on extended Huckel calculatio ns and using the fragment molecular orbital approach show that the iso mer with the three medium Pt-Pt distances is slightly more stable, in agreement with earlier theoretical predictions. Cluster core isomerism remains a rare phenomenon, and the present example emphasizes the rol e and the importance of flexible phosphido bridges in stabilizing clus ters as well as the unprecedented features which carl be observed in p hosphine phosphido-rich metal clusters.