Ruthenium-methoxycarbene and -allenylidene complexes bearing 1,4,7-trimethyl-1,4,7-triazacyclononane (Me 3 Tacn) and 1,10-phenanthroline (phen), [(Me 3 Tacn)(phen)RudC(OMe)R] 2+ (R ) CH 2 Ph (1), CHdCPh 2 (2), CHdC(C 6 H 4 Cl-4) 2 (3), CHdC(C 6 H 4 Me-4) 2 ( 4)), and [(Me 3 Tacn)(phen)Rud CdCdCR 2 ] 2+ (R ) Ph (5), C 6 H 4 OMe-4 (6)) have been prepared. The molecular structures of 1(PF 6 ) 2 and 2(PF 6 ) 2 reveal Ru-C distances of 1.917(3) and 1.906(4) Å, respectively. The lowest-energy dipoleallowed absorptions for complexes 1-4 (λ max ≈ 435 nm) are assigned as d π (Ru II ) f π*(phen) metalto-ligand charge transfer (MLCT) transitions, while those for complexes 5 and 6 (λ max ) 530 and 585 nm, respectively) are assigned as metal-perturbed π-π* [RudCdCdCR 2 ] intraligand transitions. Complexes 1-4 are emissive in glassy MeOH/EtOH at 77 K: excitation at λ ) 430 nm produces emission at λ max ) 570-620 nm, which are tentatively assigned as d π (Ru II ) f π*(phen) 3 MLCT in nature. Density functional theory (DFT) calculations, charge decomposition analysis (CDA), and natural bond orbital (NBO) analysis on complexes 1, 2, 5, and 6 suggest that allenylidene ligands are better electron donors and poorer acceptors compared with methoxycarbene ligands, and the Ru-C interactions in ruthenium-allenylidene and -methoxycarbene complexes can be depicted by the polarized formulation Ru δ+ dC δand nonpolarized formulation RudC, respectively. The methoxycarbene/allenylidene rotational barriers on 1, 2, and 5 are calculated to be 8.3, 6.3, and 1.5 kcal mol -1 , respectively.

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