Reaction of (eta(5)-C(5)Me(5))Re(NO)(PPh(3))(C=CH) and Cu(OAc)(2) (1.5 equiv, 80 degrees C, pyridine) gives the orange-brown ReC4Re complex (eta(5)-C(5)Me(5))Re(NO)(PPh(3))(C=CC=C)(Ph(3)P)(ON)Re(eta(5)-C(5)Me(5)) (2, 88%) as a 50:50 diastereomer mixture. Crystallization affords (SS,RR)-2 . 2CH(2)Cl(2) and solutions enriched in (SR,RS)-2 (meso). Addition of Ag+PF6- (greater than or equal to 2 equiv) yields deep blue (SS,RR)- and (SR,RS)-2(2+)2PF(6)(-) (86%), which give two geometric isomers (62:38, 89:11; CD2Cl2, -93 degrees C) about the Re-+=C=C=C=C=Re+ linkages. Crystal structures of (SS,RR)-2 . 2CH(2)Cl(2) and (SS,RR)-(2+)2PF(6)(-) show ReC4Re units with bond angles (169-178 degrees) and lengths (C=C, 1.202(7); C-C 1.398(5); C=C 1.260-1.305(10) Angstrom) 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)-2(2+)2PF(6)(-) or Ag+PF6- (1 equiv) give a green radical cation (SS,RR)-2(.+)PF(6)(-) (mu = 1.74 mu(B); Faraday method). E(o) data yield a K-c value of 1.1 x 10(9) (CH2Cl2, 22.5 degrees 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 upsilon(NO) band, positioned between those of 2 and 2(2+)2PF(6)(-) (CH2Cl2, 1665 vs 1623/1719 cm(-1)). Near IR spectra show unique solvent-independent bands (883, 1000, 1200 nm; is an element of 15000, 9400, 3200 M(-1) cm(-1)). Sodium naphthalenide reduces (SS,RR)- or (SR,RS)-2(.+)PF(6)(-) to (SS,RR)- or (SR, RS)-2, establishing configurational stability. These and related data are analyzed in detail.