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Abstract

The reaction of the ten-vertex nido anions [6,9-C₂B₈H₁₀]^2-, [6-Me-6,9-C₂B₈H₉]^2- and [6-Ph-6,9-C₂B₈H₉]^2- with [cis-PtCl₂(PMe₂Ph)₂] in CH₂Cl₂ yields the yellow crystalline closo-structured eleven-vertex compounds [2-X-1,1-(PMe₂Ph)₂-1,2,3-PtC₂B₈H₉], where X = H (compound 1), Me (compound 2), or Ph (compound 3) in yields of 58 88%. These compounds have been characterized by multielement NMR spectroscopy and, in the case of compounds 1 and 2, by single-crystal X-ray diffraction analysis. Crystals of compound 1 were monoclinic, space group P2₁/n, with a = 947.6(1), b = 1 547.7(3), c = 1 804.0(3) pm, β = 104.78(1)°, and Z = 4, solved and refined to R (R_w) = 0.0216 (0.0234) with 4 200 unique reflections I > 2.0 σ(I). Crystals of compound 2 were monoclinic, space group I2/a (= C2/c), with a = 1 824.8(2), b = 1 016.8(2), c = 1 328.2(3) pm, β = 101.48(2)°, and Z = 4, solved and refined to R (R_w) = 0.0442 (0.0474) with 2 057 unique reflections I > 2.0 σ(I). The NMR and X-ray results reveal that the closed eleven-vertex compounds [2-X-1,1-(PR₃)₂-1,2,3-PtC₂B₈H₉] exhibit a previously unsuspected but very free contrarotational fluxionality within the constrains of their η⁶-carborane-to-metal bonding that is accompanied by a marked flexing of the η⁶-{C₂B₈H₉X} moiety between nido and arachno ten-vertex geometries and electronic characteristics. The structural and bonding implications are discussed and elucidated specifically with reference to the NMR and geometrical charcteristics of [closo-2,3- C₂B₉H₁₁], [arachno-6,9- C₂B₈H₁₄] and the [nido-6-X-6,9- C₂B₈H₉]^2- anions, together with other pertinent species such as [μ-6,9-(SnMe₂)-nido-6,9-C₂B₈H₁₀] and more generally with reference to other eleven-vertex metallaheteroborano species. The general behaviour patterns thus established are summarized.