ALKYLIDYNE(CARBABORANE) COMPLEXES OF THE GROUP-6 METALS .6. EXO-NIDO-TUNGSTACARBABORANE COMPOUNDS

Protonation of mixtures of [NEt4][Mo(= CC6H4Me-4)(CO){P(OMe)3}(eta 5 - C2B9H9Me2)] and [W(= CR) - (CO)2(eta-C5R'5)] (R = C6H4Me-4 or Me, R' = H; R = C6H4Me-4, R' = Me) with HBF4.Et2O in CH2Cl2 at -78-degrees-C affords the mononuclear tungsten complexes [exo-nido-9,10-{W(CO)2(eta-C5R'5)}-9,10-(mu-H)2-7,8-C2B9H8-7,8-Me2]. An X-ray diffraction study (R' = Me) reveals a structure which may be regarded formally as an ion pair, with a [nido-7,8-C2B9H10Me2]- anion co-ordinated to a [W(CO)2(eta-C5Me5)] + cation by two exopolyhedral three-centre two-electron B-H half-arrow-right W bonds. The boron atoms forming these linkages lie in the open pentagonal activated CCBBB face of the cage and are adjacent. The remaining B-B connectivity in this face is bridged by a hydrogen atom. Data from B-11-H-1 and B-11-B-11 correlation NMR spectroscopy of the compound are reported and are fully in agreement with the structure. Treatment of an equimolar mixture of [NEt4][W(= CC6H4Me-4)(CO)2(eta(5)-C2B9H9Me2)] and [W(= CC6H4Me-4)-(CO)2(eta-C5H5)] in CH2Cl2 at -78-degrees-C with HBF4.Et2O affords the dimetal complex [W2(nu-CC6H4Me-4)-(CO)3(eta(5)-C2B9H9Me2)(eta-C5H5)]. In contrast, if the salt [PPh4][W(= CC6H4Me-4)(CO)2(eta(5)-C2B9H11)] is employed in a similar reaction, carried out at room temperature the product obtained, as shown by NMR spectroscopy, exists in solution as an equilibrium mixture (ca. 10:1) of two isomers, the major species being highly fluxional. Crystals of the latter were obtained from solution, and an X-ray diffraction analysis revealed the complex to be [exo-nido-9,11-(CH2C6H4Me-4)2-5,10-{W(CO)2(eta-C5H5)}-5,10-(nu-H)2-7,8-C2B9H8]. In this unusual structure the two boron atoms adjacent to the carbons in the open activated CCBBB face of the nido-C2B9 anion carry CH2C6H4Me-4 substituents. The remaining boron atom forms a B-H half-arrow-right W bridge to the exopolyhedral [W(CO)2(eta-C5H5)]+ fragment, which is also attached to the cage through a second B-H half-arrow-right W linkage, involving a boron atom in the pentagonal boron layer below the face. The NMR data suggest that the minor isomer existing in solution is a closo-icosahedral species [WH(CO)2{eta(3)-7,8-C2B9H9-9,11-(CH)2C6H4Me-4)2}(eta-C5H5)].