Five-membered titana- and zirconacyclocumulenes: Stable 1-metallacyclopenta-2,3,4-trienes

The rather exotic and uncommon five-membered metallacyclocumulenes (1-metallacyclopenta-2,3,4-trienes) CP'M-2(eta(4)-RC4R) (Cp' = substituted eta(5) -cyclopentadienyl) are formed by "cis" complexation of titanocene or zirconocene fragments with 1,3-butadiynes or by coupling of two sigma-acetylide groups in the coordination sphere of the metals. Examples of these complexes, CP2M(eta(4)-RC4R) (M = Zr, R = t-Bu; M = Ti, R = t-Bu, R = Ph), CP*M-2(eta(4) -RC4R') (M = Zr, R = R' = Me3Si, Me, Ph, t-BuMe2Si; R = t-Bu, R' = CdropC-t-Bu), (eta(6)-C5H5BX)(2)Zr(eta(4)- RC4R) (X = i-Pr2N, R = Ph; X = Ph, R = Ph, Et), (eta(5):eta(1) -C5Me4SiMe2N-t-Bu)Ti(eta(4)-RC4R) (R = t-Bu, SiMe3 and [CP2Zr](3)[1,3,5-(eta(4) -t-BuC4)(3)C6H3], are discussed with regard to their genesis, synthesis, dynamics, structures, and bonding. These compounds are compared with the also uncommon and very similar five-membered metallacyclopentynes (1-metallacyclopent-3-ynes) CP'Mo-2(eta(4)-R2C4R2) (M = Zr: Cp'= Cp, R-2 = t-Bu/H, Me3Si/H, H-2; Cp'= t-BuC5H4, R-2 = H-2; M = Ti: Cp' = Cp, R-2 = H-2) formed by analogous "cis" complexation of 1,2,3-butatrienes, in terms of their chemistry, structures, and bonding. Furthermore, reactions in which metallacyclocumulenes were assumed to be intermediates are described, together with the reactivity of metallacyclocumulenes toward different complexes and substrates: e.g. metal complexes, carbon dioxide, diisobutylaluminum hydride, and tris(pentafluorophenyl)borane. Upon complexation with a second transition-metal complex fragment either bridging formal mu-"cis" mu-"trans" complexes of 1,3-butadiynes and 1,2,3-butatrienes were formed.