Dihydrobis(pyrazolyl)borate Alkylidyne Complexes of Tungsten

[W(≡CR)Br(CO) 2 (NC 5 H 4 Me‐4) 2 ] (R = C 6 H 3 Me 2 ‐2,6 1a , C 6 H 2 Me 3 ‐2,4,6 1b ) react with K[H 2 B(pz) 2 ] (pz = pyrazol‐1yl) to provide two interconverting isomers of [W(≡CR)(CO) 2 (NC 5 H 4 Me‐4){H 2 B(pz) 2 }] ( 2 ), one isomer having been structurally characterised for R = C 6 H 3 Me 2 ‐2,6 ( 2a ). The reactions of [W(≡CR)Br(CO) 2 (CNR′) 2 ] ( 5 : R′ = C 6 H 3 Me 2 ‐2,6, CNMe 3 ) with K[H 2 B(pz) 2 ] or of [W(≡CR)Br(CO) 2 (NC 5 H 4 Me‐4){H 2 B(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(≡CR)Br(CO) 2 (CNR′) 2 ] ( 5 ) with K[H 2 B(pz) 2 ] provides [W(≡CR)(CO) 2 (CNR′){H 2 B(pz) 2 }] ( 3 ) in moderate yield, similar yields were obtained from [W(≡CR)(CO) 2 (NC 5 H 4 Me‐4){H 2 B(pz) 2 }] ( 2 ) and CNR′. Under mild photolytic conditions both combinations provide primarily the thermally unstable ketenyl complex [W(η 2 ‐OCCR)(CO)(CNR′) 2 {H 2 B(pz) 2 }] ( 4 ). The picoline ligand in [W(≡CR)Br(CO) 2 (NC 5 H 4 Me‐4){H 2 B(pz) 2 }] ( 2 ) is readily replaced by PMe 2 Ph to provide [W(≡CR)(CO) 2 (PMe 2 Ph){H 2 B(pz) 2 }] (structurally characterised for R = C 6 H 3 Me 2 ‐2,6 6a ). The reaction of either [W(≡CR)(CO) 2 (PMe 2 Ph){H 2 B(pz) 2 }] ( 6 ) or [W(≡CR)(CO) 2 (NC 5 H 4 Me‐4){H 2 B(pz) 2 }] (R = C 6 H 2 Me 3 ‐2,4,6 2b ) with excess PMe 2 Ph provides [W(≡CR)(CO)(PMe 2 Ph) 2 {H 2 B(pz) 2 }] ( 7 ). This complex is also obtained from K[H 2 B(pz) 2 ] and [W(≡CR)Br(CO)(PMe 2 Ph) 3 ] ( 8 ), the latter arising from the reaction of [W(≡CR)Br(CO) 2 (NC 5 H 4 Me‐4) 2 ] ( 1b ) with PMe 2 Ph. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)