POLYHEDRAL OSMABORANE CHEMISTRY - THE PLATINATION OF [4,4,4-(CO)(PPH3)2-NIDO-4-OSB5H9] AND THE ISOLATION, NUCLEAR-MAGNETIC-RESONANCE PROPERTIES, AND MOLECULAR-STRUCTURES OF 2 7-VERTEX NIDO-TYPE DIMETALLAHEPTABORANES [(CO)(PPH3)2HOS(PME2PH)CLPTB5H7] AND [(CO)(PPH3)(PPH2)OS(PPH3)PTB5H7PH]

Reaction of the nido six-vertex osmaborane [(CO)(PPh3)2OsB5H9] 1 with base followed by [PtCl2(PMe2Ph)2] yields (34%) the air-stable lemon-yellow seven-vertex osmaplatinaborane [(CO)-(PPh3)2 activated HOs(PMe2Ph)ClPtB5H7] 2 which retains the approximate structure of the nido six-vertex osmaborane 1, but which has three basal atoms (BBOs) co-ordinated trihapto to a {PtCl(PMe2Ph)} moiety, with the platinum and osmium atoms linked by a two-electron hydrogen bridge. The overall seven-vertex structure resembles that of a regular triangulated dodecahedron that has a five-connected vertex missing. It is geometrically and electronically nido. Treatment of this species with base, or thermolysis at 140-degrees-C, gives a small yield (3%) of the related species [(CO)(PPh3) activated (PPh2)Os(PPh3)PtB5H7Ph] 3. This is formally derived from 2 via PPh3 half-arrow-pointing-left and half-arrow-pointing-right PMe2Ph ligand exchange plus the elimination of HCl, and has a similar nido-type seven-vertex osmaplatinaheptaborane cluster geometry. Now however there is a direct osmium-platinum bond, the metals are also linked by a four-electron {PPh2} bridge, and a migrant phenyl group is bound to a basal boron atom on the nido-{OsB5} subcluster. The compounds have been investigated by NMR spectroscopy, the results of which suggest that a number of other (minor) products from the reactions may also be interesting platinaosmaboranes. Crystals of 2 are monoclinic, space group P2(1)/n, with a = 1.5397(3), b = 1.8489(3), c = 1.7189(3) nm, beta = 92.568(13)-degrees and Z = 4. Crystals of 3 are triclinic, space group P1BAR (no. 2), with a = 1.4491(7), b = 1.7730(4), c = 1.0763(3) nm, alpha = 82.08(2), beta = 102.71(3), gamma = 105.42(4)-degrees and Z = 2.