11-VERTEX POLYHEDRAL DICARBAPLATINABORANE CHEMISTRY - ASPECTS OF THE CHEMISTRY OF SOME CLOSO-STRUCTURED (1,2,3-PTC2B8) SPECIES AND SOME RELATED-COMPOUNDS

The reaction of the ten-vertex nido anions [6,9-C2B2H10]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 P2(1)/n, with a = 947.6(l), b = 1 547.7(3), c = 1 804.0(3) pm, beta = 104.78(1)-degrees, and Z = 4, solved and refined to R (R(W)) = 0.0216 (0.0234) with 4 200 unique reflections I > 2.0 sigma(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, beta = 101.48(2)-degrees, and Z = 4, solved and refined to R (R(w)) = 0.0442 (0.0474) with 2 057 unique reflections I > 2.0 sigma(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 constraints of their eta6-carborane-to-metal bonding that is accompanied by a marked flexing of the eta6-{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 characteristics 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 [mu-6,9-(SnMe2)-nido-6,9-C2B8H10], and more generally with reference to other eleven-vertex metallaheteroborane species. The general behavioural patterns thus established are summarized.