Two transition-metal hydride complexes of the type M(dippe)(2)(NO)(H)] M = W (2a), Mo (2b); dippe = 1,2-bis(di-isopropylphosphanyl)ethane] have been prepared by the reaction of M(dippe)(2)(NO)(Cl)] M = W (1a), Mo (1b)] with LiBH4. The nitrosyl groups of the tungsten complexes la and 2a are capable to coordinate a LiBH4 molecule to form the stable adducts W(dippe)(2)(Cl)(NO center dot center dot center dot LiBH4) (1c) and W(dippe)(2)-(H)(NO center dot center dot center dot LiBH4) (2c). Addition of ethylenediamine to a toluene solution of 2c led to rupture of the 2c adduct and afforded 2a in good yield. After the interaction of 2a with H(Et2O)]BF4], the stable seven-coordinated cationic dihydride W(dippe)(H)(2)(dippe)(NO)]BF4] (6a) was isolated. The reaction between 2b and H(Et2O)]BF4] led to the formation of Mo(dippe)(2)(NO)(FBF3)] (6b) in which BF4- was found to be coordinated to the metal centre. The H(Et2O)(2)]BAr4F] Ar-F = 3,5-(CF3)(2)C6H3] acid interacted with 2a to yield the seven-coordinated complex W(dippe)(H)(2)(dippe)(NO)]-BAr4F] (5a) similar to 6a. In the reaction between 2b and H(Et2O)(2)]BAr4F], the 16e(-) five-coordinated complex Mo-(dippe)(2)(NO)]BAr4F] (4b) was formed. X-ray diffraction revealed that 4a has a weak agostic interaction trans to the NO ligand. Complex 4b was found to react rapidly with hydrogen gas under ambient conditions to form the dihydride complex Mo(dippe)(H)(2)(dippe)(NO)]BAr4F] (5b), which is unstable in the absence of a hydrogen atmosphere. The equilibrium constant for the reversible reaction of 4b with hydrogen was found to be K = 2.6 bar(-1) at 25 degrees C. Complex 4b was tested as a catalyst for acetone hydrogenation; a maximum TON of 7 was found.