Polyhedral oxaplatinaborane chemistry:: characterisation of [9,9-(PMe2Ph)2-arachno-9,6-PtOB8H10] and its metallaborane non-oxa cognate [6,6-(PMe2Ph)2-arachno-6-PtB9H11-9-(PMe2Ph)]

Reaction of [(PMe2Ph)(2)PtB8H12] with KH and [{(eta(5)-C5Me5)RhCl2}(2)] followed by chromatographic separation in air unexpectedly gave the ten-vertex oxametallaborane [9,9-(PMe2Ph)(2)-arachno-9,6-PtOB8H10] (1, 11%), which has a classical arachno ten-vertex geometry with the {O} and {Pt(PMe2Ph)(2)}, vertices in the prow (6)- and (9)-positions; the oxygen atom is therefore bound contiguously to boron atoms only. Reaction of [PtMe2(PMe2Ph)(2)] with [6,9-(PMe2Ph)(2)B10H12] generates its ten-vertex metallaborane cognate [6,6-(PMe2Ph)(2)-arachno-6-PtB9H11-9-(PMe2Ph)] (3, 10%), and completes a coherent B-->C-->N-->O cluster constituent sequence. Compounds 1 and 3 are characterised by single-crystal X-ray diffraction analysis and NMR spectroscopy. (C) 1998 Elsevier Science S.A.