Abstract : Carbyne complexes have been extensively studied but only a few of them contain the M[triple bond, length as m-dash]C–H moiety (M = transition metal). In order to contribute to the debate about the linearity of this fragment, two neutron diffraction experiments on single crystal have been performed at 100 and 293 K on the trans-W(CH)(dmpe)2Br (dmpe = 1,2-bis(dimethylphosphino)ethane) (1). The results point, without ambiguity, to the linearity of the fragment. The specific behaviour of the dmpe ligand is modelled and interpreted.