Spectroscopic Studies and Structures of trans-Ruthenium(II) and Ruthenium(III) Bis(cyanide) Complexes Supported by a Tetradentate Macrocyclic Tertiary Amine Ligand

trans-[Ru(16-TMC)(CN)₂] (1; 16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane) was prepared by the reaction of trans-[Ru(16-TMC)Cl₂]Cl with KCN in the presence of zinc powder. The oxidation of 1 with bromine gave trans-[Ru(16-TMC)(CN)₂]⁺ isolated as PF₆ salt (2·PF₆). The Ru−C/C−N distances are 2.061(4)/1.130(5) and 2.069(5)/1.140(7) Å for 1 and 2, respectively. Both complexes show a Ru(III/II) couple at 0.10 V versus FeCp₂^+/0. The UV−vis absorption spectrum of 1 is dominated by an intense high-energy absorption at λ_max = 230 nm, which is mainly originated from d_π(Ru^II) → π*(NC−Ru−CN) charge-transfer transition. Complex 2 shows intense absorption bands at λ_max ≤ 228 nm and weaker vibronically structured absorption bands with peak maxima at 315−441 nm (ε_max ≈ (5−8) × 10² dm³ mol⁻¹ cm⁻¹), which are assigned to d_π(Ru^III) → π*(NC−Ru−CN) and σ(⁻CN) → d(Ru^III) charge-transfer transition, respectively. Density functional theory and time-dependent density-functional theory calculations have been performed on trans-[(NH₃)₄Ru(CN)₂] (1′) and trans-[(NH₃)₄Ru(CN)₂]⁺ (2′) to examine the Ru−cyanide interaction and the nature of associated electronic transition(s). The 230 nm band of 1 has been probed by resonance Raman spectroscopy. Simulations of the absorption band and the resonance Raman intensities show that the nominal ν_CN stretch mode accounts for ca. 66% of the total vibrational reorganization energy. A change of nominal bond order for the cyanide ligand from 3 to 2.5 is estimated upon the electronic excitation.