Dihydrobis(pyrazolyl)borate alkylidyne complexes of tungsten

[W(dropCR)Br(CO)(2)(NC5H4Me-4)(2)] (R = C6H3Me2-2,6 1a, C6H2Me3-2,4,6 1b) react with K[H2B(pz)(2)] (pz = pyrazol-1-yl) to provide two interconverting isomers of [W(dropCR)(CO)(2)(NC5H4Me-4){H2B(pz)(2)}] (2), one isomer having been structurally characterised for R = C6H3Me2-2,6 (2a). The reactions of [W(dropCR)Br(CO)(2)(CNR')(2)] (5: R' = C6H3Me2-2,6, CNMe3) with K[H2B(pz)(2)] or of [W(dropCR)Br(CO)(2)(NC5H4Me-4){H2B(pz)(2)}] (2) with isocyanides are more complex due to the facile formation of unstable ketenyl derivatives. In the absence of light, the reaction of [W(dropCR)Br(CO)(2)(CNR')(2)] (5) with K[H2B(pz)(2)] provides [W(dropCR)(CO)(2)(CNR'){H2B(pz)(2)}] (3) in moderate yield, similar yields were obtained from [W(dropCR)(CO)(2)(NC5H4Me-4){H2B(pz)(2)}] (2) and CNR'. Under mild photolytic conditions both combinations provide primarily the thermally unstable ketenyl complex [W(eta(2)-OCCR)(CO)(CNR')(2){H2B(pz)(2)}] (4). The picoline ligand in [W(dropCR)Br(CO)(2)(NC5H4Me-4){H2B(pz)(2)}] (2) is readily replaced by PMe2Ph to provide [W(dropCR)(CO)(2)(PMe2Ph){H2B(pz)(2)}] (structurally characterised for R = C6H3Me2-2,6 6a). The reaction of either [W(dropCR)(CO)(2)(PMe2Ph){H2B(pz)(2)}] (6) or [W(dropCR)(CO)(2)(NC5H4Me-4){H2B(pz)(2)}] (R = C6H2Me3-2,4,6 2b) with excess PMe2Ph provides [W(dropCR)(CO)(PMe2Ph)(2){H2B(pz)(2)}] (7). This complex is also obtained from K[H2B(pz)(2)] and [W(dropCR)Br(CO)(PMe2Ph)(3)] (8), the latter arising from the reaction of [W(dropCR)Br(CO)(2)(NC5H4Me-4)(2)] (1b) with PMe2Ph. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.