Steric effects in heteroboranes. Part 9. Cycloocta-1,5-diene palladium and platinum complexes of mono- and di-phenyl C₂B₉ ligands: Metal-mediated polyhedral rearrangement

The compounds 1-Ph-3-(cod)-3,1,2-PdC₂B₉H₁₀ 1a, 1,2-Ph₂-3-(cod)-3,1,2-PdC₂B₉H₉ 2a, 1-Ph-3-(cod)-3,1,2-PtC₂B₉H₁₀ 3a, and 1,11-Ph₂-3-(cod)-3,1,11-PtC₂B₉H₉ 4, have been prepared from the reactions between TI₂[7-Ph-nido-7,8-C₂B₉H₁₀] or TI₂[7,8-Ph₂-nido-7,8-C₂B₉H ₉] and [PdCl₂(cod)] or [PtCl₂(cod)] as appropriate (cod = cycloocta-1,5-diene). The platinum reactions are considerably cleaner than the palladium ones. Compound 2a was subjected to a crystallographic study which reveals severe intramolecular overcrowding; the palladium atom is slipped by 0.56 Å away from the C-bound phenyl groups, the co-ordination of the cod ligand to the metal is distorted and the cage is deformed with extended C-B distances in the ligand face. Although 1a and 3a are structural analogues, 2a and 4 are not. Rather, compound 4 is only isolated as the C-separated species defined by a 3,1,11-PtC₂B₉ architecture, thus illustrating that the nature of the metal atom can influence the ease with which carbametallaboranes undergo polyhedral rearrangement. Compound 1a was recovered unchanged from refluxing toluene, but under the same conditions 2a was converted into 2a', tentatively identified as the rearranged isomer 1,11-Ph₂-3-(cod)-3,1,11-PdC₂B₉H₉.