The reaction of [MoBr(dppe)(η-C 7 H 7 )] (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) with HC≡CC≡CSiMe 3 and Na[BPh 4 ] in 1:1 NHEt 2 /thf as solvent, yields the aminoallenylidene complex [Mo{C=C=C(Me)NEt 2 }(dppe)(η-C 7 H 7 )]BPh 4 , [1]BPh 4 . The reaction likely proceeds via nucleophilic addition of NHEt 2 at C γ of a butatrienylidene intermediate. Structural and spectroscopic characterisation of [1]BPh 4 indicate a significant contribution of an iminiumalkynyl resonance form to the overall structure of the heteroatom stabilised allenylidene ligand. The X-ray structural study of [1]BPh 4 determines a Mo-C α bond length of 2.077(3) Å, intermediate between that of the cumulenic diphenylallenylidene analogue [Mo(C=C=CPh 2 )(dppe)(η-C 7 H 7 )]PF 6 (1.994(3)Å) and the alkynyl [Mo(C≡CPh)(dppe)(η-C 7 H 7 )] (2.138(5)Å). Complex [1]BPh 4 undergoes a reversible one-electron oxidation with E ½ = -0.19 V with respect to the FeCp 2 /FeCp 2 + couple and the stable 17-electron radical dication [1] 2+ is readily observed by spectroelectrochemical methods. IR spectroelectrochemistry in CH 2 Cl 2 demonstrates that the ν(CCC) stretch, characteristic of the allenylidene ligand, shifts to higher wavenumber (from 1959 to 2032 cm -1 ) as a result of oxidation of [1] + to [1] 2+ , consistent with a strongly metal-centred redox process and an enhancement in the alkynyl character of the allenylidene ligand following one-electron oxidation.