Protonation of mixtures of [NEt,] [Mo(=CC,H,Me-4) (CO){P(OMe),}(q5-C,BsH9Me2)] and [W(=CR)-(CO),(q-C,R',)] (R = C,H,Me-4 or Me, R' = H; R = C,H,Me-4, R' = Me) with HBF,-Et,O in CH,CI, at -78 "C affords the mononuclear tungsten complexes [exo-nidu-9,1 O-{W(CO),(q-C,R',))-9,10-(p-H),-7,8-C2B9H,-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-C,B9Hl,Me,] -anion co-ordinated to a [W(CO),(q-C,Me,)] + cation by two exopolyhedral three-centre two-electron B-H-W bonds. The boron atoms forming these linkages lie in the open pentagonal face of the cage and are adjacent. The remaining B-B connectivity in this face is bridged by a hydrogen atom. Data from llB-'H and lrB-llB correlation NMR spectroscopy of the compound are reported and are fully in agreement with the structure. Treatment of an equimolar mixture of [NEt,] [W(=CC,H,Me-4) (CO),(q5-C,BsH9Me2)] and [W(=CC,H,Me-4)-( C0),(q-C5H5)] in CH,CI, at -78 "C with HBF,-Et,O affords the dimetal complex [W,(p-CC,H,Me-4)-(CO),(q5-C,B9H9Me,) (q-C5H5)]. In contrast, if the salt [PPh,] [W(GCC6H,Me-4) (CO),(q5-C,B9Hll)] is employed in a similar reaction, carried out at room temperature, the product obtained, as shown by N M R spectroscopy, exists in solution as an equilibrium mixture (ca. 1 O : l ) 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-nidu-9,11 -(CH,C,H,Me-4),-5,l O-{W(CO),(q-C5H,)}-5,l 0-(p-H),-7,8-C,B9H,]. In this unusual structure the two boron atoms adjacent to the carbons in the open face of the nido-CZB9 anion carry CH,C,H,Me-4 substituents. The remaining boron atom forms a B-H-W bridge to the exopolyhedral [W(CO),(q-C,H5)] + fragment, which is also attached to the cage through a second B-H-W linkage, involving a boron atom in the pentagonal boron layer below the face. The N M R data suggest that the minor isomer existing in solution is a closo-icosahedral species [WH (C0),{q3-7,8-C2B9H9-9,1 1 -(CH,C,H,Me-4),}(1.1-c~H5)]. Treatment of the anionic complexes [W(=CR)(C0),(qs-7,8-C,B,H,Me,)]-(R = alkyl or aryl) in CH,CI, at ca. -78 "C with HBF,*Et,O in the presence of donor molecules (CO, CNBu', tertiary or secondary phosphines, alkynes, etc.) affords a variety of mononuclear tungsten compounds with novel structures.'S2 In many of the products the metal atom is q5 ligated by a nido-C,B,H,(CH,R)Me, icosahedral fragment. The CH,R groups in the latter evidently arise via insertion of an initially formed tungsten-bonded alkylidene moiety into a B-H bond lying in the open pentagonal face of the carbaborane cage. Invariably the boron atom carrying the CH,R substituent is in the p site with respect to the two carbon atoms. Similar experiments involving treatment of the species [M(=CR)(C0),(q5-7,8-C2B9H9Me2)]-(M = Mo or W) with HBF,*Et,O in the presence of the reagents [M(=CR)(CO),(q-C,H,)] afford dimetal compounds. However, the nature of the latter depends critically ...