The allenylidene complexes trans- [FeBr(dCdCdCRR′) (depe) 2 ][Y] (R ) Me, R′ ) Ph, 1; R ) R′ ) Ph, 2; R ) R′ ) Et, 3; depe ) Et 2 PCH 2 CH 2 PEt 2 ; Y ) BF 4 , BPh 4 ) were obtained by reaction of trans-[FeBr 2 (depe) 2 ] with the appropriate alkynol HCtCCRR′(OH), in MeOH and in the presence of Na[BF 4 ] or Na[BPh 4 ]. Deprotonation of 3 or nucleophilic γ-addition to 2 led to the neutral enynyl and alkynyl complexes trans- [FeBr{-CtCC(dCHMe)Et}-, in acetonitrile solution, upon reaction with NHMe 2 , NH 2 Me, and PMe 3 , respectively. The complexes have been characterized by multinuclear NMR and IR spectroscopy, FAB-MS, and elemental analysis and, in the cases of 5a and 6a, also by X-ray diffraction analysis. Controlled-potential electrolysis of 2 yields the alkynyl species trans-[FeBr{-CtCCPh 2 (H)}(depe) 2 ] (8) via a 2e -/H + process, and the oxidation potential of the complexes, measured by cyclic voltammetry, has allowed us to estimate the electrochemical Pickett (P L ) and Lever (E L ) ligand parameters for the cumulenic ligands. These are then ordered (together with related ligands) according to their net π-electron acceptor minus σ-donor ability as follows: carbynes > aminocarbyne > CO > vinylidenes > aryl allenylidene > alkyl allenylidene > NCR . phosphonium alkynyl > cyanoalkynyl, Br -, NCO -> alkynyl, enynyl, aminoalkynyl.