The synthesis and spectroscopic properties of trans-[Cl(16-TMC)Ru=C=CHR]PF6 (16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane, R = C6H4X-4, X = H (1), Cl (2), Me (3), OMe (4); R = CHPh2 (5)), trans-[Cl(16-TMC)Ru=C=C=C(C6H4X-4)(2)]PF6 (X = H (6), Cl (7), Me (8), OMe (9)), and trans-[Cl(dppm)(2)M=C=C=C(C6H4X-4)(2)]PF6 (M = Ru, X = H (10), Cl (11), Me (12); M = Os, X = H (13), Cl (14), Me (15)) are described. The crystal structures of 1, 5, 6, and 8 show that the Ru-C-alpha and C-alpha-C-beta distances of the allenylidene complexes fall between those of the vinylidene and acetylide relatives. Two reversible redox couples are observed by cyclic voltammetry for 6-9, with E-1/2 values ranging from -1.19 to -1.42 and 0.49 to 0.70 V vs Cp2Fe+/0, and they are both 0.2-0.3 and 0.1-0.2 V more reducing than those for 10-12 and 13-15, respectively. The UV-vis spectra of the vinylidene complexes 1-4 are dominated by intense high-energy bands at lambda(max) less than or equal to 310 nm (epsilon(max) greater than or equal to 10(4) dm(3) mol(-1) cm(-1)), while weak absorptions at lambdamax greater than or equal to 400 nm (epsilon(max) less than or equal to 10(2) dm(3) mol(-1) cm(-1)) are tentatively assigned to d-d transitions. The resonance Raman spectrum of 5 contains a nominal v(C=C) stretch mode of the vinylidene ligand at 1629 cm(-1). The electronic absorption spectra of the allenylidene complexes 6-9 exhibit an intense absorption at lambda(max) = 479-513 nm (epsilon(max) = (2-3) x 10(4) dm(3) mol(-1) cm(-1)). Similar electronic absorption bands have been found for 10-12, but the lowest energy dipole-allowed transition is blue-shifted by 1530-1830 cm(-1) for the Os analogues 13-15. Ab initio calculations have been performed on the ground state of trans-[Cl(NH3)(4)Ru=C=C=CPh2](+) at the MP2 level, and imply that the HOMO is not localized purely on the metal center or allenylidene ligand. The absorption band of 6 at lambda(max) = 479 nm has been probed by resonance Raman spectroscopy. Simulations of the absorption band and the resonance Raman intensities show that the nominal v(C=C=C) stretch mode accounts for ca. 50% of the total vibrational reorganization energy, indicating that this absorption band is strongly coupled to the allenylidene moiety. The excited-state reorganization of the allenylidene ligand is accompanied by rearrangement of the Ru=C and Ru-N (of 16-TMC) fragments, which supports the existence of bonding interaction between the metal and C=C=C unit in the electronic excited state.