The reactions of 2-borane-1,3,4,5-tetramethylimidazoline (BH3 center dot NHCMe) with selected phosphane adducts of the Lewis acids B(C6F5)(3) and Ga(C6F5)(3) are Studied. Among others, adducts (C6F5)(3)Ga center dot PH2Cp* (1a) and (C6F5)(3)B center dot PH2Cp* (2) are used as starting materials. When the (C6F5)(3)Ga-phosphane adducts 1a and (C6F5)(3)Ga center dot PPhH2 are treated with BH3 center dot NHCMe, the Lewis acid/base stabilised phosphanylboranes (C6F5)(3)Ga center dot P(Cp*)HBH2 center dot NHCMe (3a) and (C6F5)(3)Ga center dot P(Ph)HBH2 center dot NHCMe (3b) are formed, respectively, by a hydrogen elimination reaction. In contrast, the reaction of BH3 center dot NHCMe with the (C6F5)(3)B-phosphane adducts (C6F5)(3)B center dot PH2R [R = H, R = Cp* (2) and R = Ph] in CH2Cl2 at room temperature leads to the formation of a salt with the general formula [(C6F5)(3)BH][RPH2 center dot BH2 center dot NHCMe] (4a: R = H, 4b: R = Cp-center dot, 4c: R = Ph). To synthesise the Lewis acid/base stabilised phosphanylborane with (C6F5)(3)B as a Lewis acid and 1,3,4,5-tetramethylimidazolylidene (NHCMe) as a Lewis base, a different synthetic pathway was applied: the replacement reaction of the Lewis base. At room temperature, NHCMe displaced the amine in (C6F5)(3)B center dot P(Ph)HBH2 center dot NMe3 to yield (C6F5)(3)B center dot P(Ph)HBH2 center dot NHCMe (5). All compounds were comprehensively characterised by spectroscopic methods. Compounds la, 1b, 2, 3a, 3b and 5 were additionally characterised by X-ray crystallographic analysis. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451. Weinheim, Germany, 2008).