The allenylidene complexes trans-[FeBr(CCCRR‘)(depe)2][Y] (R = Me, R‘ = Ph, 1; R = R‘ = Ph, 2; R = R‘ = Et, 3; depe = Et2PCH2CH2PEt2; Y = BF4, BPh4) were obtained by reaction of trans-[FeBr2(depe)2] with the appropriate alkynol HCCCRR‘(OH), in MeOH and in the presence of Na[BF4] or Na[BPh4]. Deprotonation of 3 or nucleophilic γ-addition to 2 led to the neutral enynyl and alkynyl complexes trans-[FeBr{−CCC(CHMe)Et}(depe)2] (4) and trans-[FeBr(−CCCPh2R‘ ‘)(depe)2] (R‘ ‘ = CN (5a), MeO (5b)), respectively. Complex 2 (Y = BPh4) also leads to the cationic alkynyl compounds trans-[Fe(NCMe){−CCCPh2(X)}(depe)2][BPh4] (X = NMe2 (6a), NHMe (6b)) and trans-[Fe(NCMe){−CCCPh2(PMe3)}(depe)2]Y2 (Y2 = [BPh4]2 (7a), [BPh4]2-xBrx (7b)), in acetonitrile solution, upon reaction with NHMe2, NH2Me, and PMe3, 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{−CCCPh2(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 (PL) and Lever (EL) 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.