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 Cβ and Cδ in the model complex [Mo­(CCCCH)­(dppe)­(η-C7H7)]+.