Synthesis, Redox Chemistry, and Electronic Structure of the Butadiynyl and Hexatriynyl Complexes [Mo{(CC)_nCCR}(L₂)(η‑C₇H₇)]^z+ (n = 1, 2; z = 0, 1; R = SiMe₃, H; L₂ = 2,2′-bipyridine, Ph₂PCH₂CH₂PPh₂)

Two series of extended carbon chain butadiynyl and hexatriynyl complexes, [Mo­{(CC)_nCCSiMe₃}­(bpy)­(η-C₇H₇)] (n = 1, 2; bpy = 2,2′-bipyridine) and [Mo­{(CC)_nCCR}­(dppe)­(η-C₇H₇)] (n = 1, R = H, SiMe₃; n = 2, R = SiMe₃; dppe = Ph₂PCH₂CH₂PPh₂), have been prepared and structurally characterized. The redox chemistry of these complexes has been investigated by cyclic voltammetry, and the 17-electron radical cations resulting from one-electron oxidation have been characterized by spectroelectrochemical IR and UV–visible methods and EPR spectroscopy. DFT calculations on the H-terminated model complexes [Mo­{(CC)_nCCH}­(L₂)­(η-C₇H₇)]^z+ (L₂ = bpy, dppe) reveal a largely metal-centered HOMO (z = 0) with a modest increase in carbon chain character with increasing chain length. Spin density calculations for the 17-electron radical cations (z = 1) show large coefficients of spin density at the metal center, consistent with the remarkably high stability of the experimental complexes. However, both DFT theoretical and experimental synthetic studies highlight a distinction between the bpy- and dppe-supported systems. The 17-electron complexes [Mo­{(CC)_nCCSiMe₃}­(bpy)­(η-C₇H₇)]­PF₆ (n = 1, 2) are unique examples of isolable, metal-stabilized butadiynyl and hexatriynyl radicals. In contrast, the dppe radical [Mo­(CCCCSiMe₃)­(dppe)­(η-C₇H₇)]⁺ exhibits chain-centered reactivity, consistent with enhanced coefficients of spin density at C_β and C_δ in the model complex [Mo­(CCCCH)­(dppe)­(η-C₇H₇)]⁺.