Iron and Ruthenium σ‑Polyynyls of the General Formula [{M(dppe)Cp*}–(CC)n–R]0/+ (M = Fe, Ru): An Experimental and Theoretical Investigation

Two series of metal-polyynyl complexes of iron and ruthenium of general formula [{M­(dppe)­Cp*}–(CC)n–R]0/+ (M = Fe, Ru; R = H, Ph, SiMe3, Au­(PPh3); n = 1–3), have been synthesized, characterized, and theoretically analyzed. The results provide a comprehensive description of the effect of the length of the conjugated carbon chain and the role of the nature of the metal atom and the terminal substituent on their neutral and oxidized states. For the latter, the spin density found on the carbon chain is a source of instability; e.g., for R = Au­(PPh3), the oxidized compounds are much more accessible electrochemically than the rest of the series but are susceptible to radical attack. Of particular interest is the use of joint experimental and theoretical EPR studies, which allow elucidation of the differences of behavior within the two series. It reveals that the atomic spin density on the metal is not a sufficient criterion to evaluate EPR anisotropy but that the specific nodal properties of the frontier spin–orbitals highly influence the EPR components. The localization of the spin density on specific carbon atoms of the conjugated chain (even numbered) opens up the possibility of building extended systems by targeted radical reactions.