Consanguineous families of coordinated carbon: A ReC₄Re assembly that is isolable in three oxidation states, including crystallographically characterized ReC ≡ CC ≡ CRe and ⁺Re = C = C = C = C = Re⁺ adducts and a radical cation in which charge is delocalized between rhenium termini

Reaction of (η⁵-C₅Me₅)Re(NO)(PPh₃)(C ≡ CH) and Cu(OAc)₂ (1.5 equiv, 80 °C, pyridine) gives the orange-brown ReC₄Re complex (η⁵-C₅Me₅)Re(NO)(PPh₃)(C ≡ CC ≡ C)(Ph₃P)(ON)Re(η⁵-C₅Me₅) (2, 88%) as a 50:50 diastereomer mixture. Crystallization affords (SS,RR)-2 · 2CH₂Cl₂ and solutions enriched in (SR,RS)-2 (meso). Addition of Ag⁺PF₆^- (≤ 2 equiv) yields deep blue (SS,RR)- and (SR,RS)-2²⁺2PF₆^- (86%), which give two geometric isomers (62:38, 89:11; CD₂Cl₂, -93 °C) about the ⁺Re = C = C = C = C = Re⁺ linkages. Crystal structures of (SS,RR)-2 · 2CH₂Cl₂ and (SS,RR)-2²⁺2PF₆^- show ReC₄Re units with bond angles (169-178°) and lengths (C ≡ C, 1.202(7); C-C 1.398(5); C = C 1.260-1.305(10) Å) near those of butadiyne or cumulenes. Natural bond order analysis and topological electron density calculations confirm the valence formulations. Reactions of (SS,RR)-2 with (SS,RR)-2²⁺2PF₆^- or Ag⁺PF₆^- (1 equiv) give a green radical cation (SS,RR)-2^.+PF₆^- (μ = 1.74 μ(B); Faraday method). E°data yield a K(c) value of 1.1 x 10⁹ (CH₂Cl₂, 22.5 °C) for the comproportionation. ESR spectra show undecets with A(iso,Re) values (98 G) half those of related monorhenium radical cations, indicating spin delocalization over two rheniums (I = 5/2). Accordingly, IR spectra give only one υ(NO) band, positioned between those of 2 and 2²⁺2PF₆^- (CH₂Cl₂, 1665 vs 1623/1719 cm^-1). Near IR spectra show unique solvent-independent bands (883, 1000, 1200 nm; ε 15000, 9400, 3200 M^-1 cm^-1). Sodium naphthalenide reduces (SS,RR)- or (SR,RS)-2^.+PF₆^- to (SS,RR)- or (SR,RS)-2, establishing configurational stability. These and related data are analyzed in detail.