Synthetic, Structural, Spectroscopic, and Theoretical Studies of Structural Isomers of the Cluster Pt3(μ-PPh2)3Ph(PPh3)2. A Unique Example of Core Isomerism in Phosphine Phosphido-Rich Clusters

Two isomers of the phosphido-bridged platinum cluster Pt3(μ-PPh2)3Ph(PPh3)2 (2 and 3) have been isolated, and their structures have been solved by single-crystal X-ray diffraction. Compound 2 crystallizes in the orthorhombic space group Cmc21 with a = 22.192(10) Å, b = 17.650(9) Å, c = 18.182(8) Å, and Z = 4. Compound 3 crystallizes with 2 molecules of dichloromethane in the monoclinic space group C2/c with a = 21.390(10) Å, b = 18.471(9) Å, c = 19.021(11) Å, β = 105.27(5)°, and Z = 4. The two isomers differ essentially in their metal−metal distances and Pt−(μ-PPh2)−Pt angles. Thus 2, having an imposed Cs symmetry, contains a bent chain of metal atoms with two short Pt−Pt distances of 2.758(3) Å and a long separation of 3.586(2) Å. In 3, which has an imposed C 2 symmetry, the metal atoms form an isosceles triangle with two Pt−Pt distances of 2.956(3) Å and one of 3.074(4) Å. These isomers can be smoothly interconverted by changing the crystallization solvents. Solution and solid-state 31P NMR studies have been performed in order to assign the resonances of the different P nuclei and relate their chemical shifts with their structural environments. Raman spectroscopy was used to assign the ν(Pt−Pt) modes of the two structural isomers. Theoretical studies based on extended Hückel calculations and using the fragment molecular orbital approach show that the isomer 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 role and the importance of flexible phosphido bridges in stabilizing clusters as well as the unprecedented features which can be observed in phosphine phosphido-rich metal clusters.