Synthesis and characterization of [Cp2V(mu-eta(2):eta(4)-butadiyne)ZrCp ' 2] heterodimetallic complexes (Cp ' = C(5)H(4)t-Bu, C5H4Me). Formation mechanism and theoretical (ELF) evidence for the existence of planar tetracoordinate carbon (ptC)

Reaction of (C5H4R)(2)Zr(C=CPh)(2) (R = Me, t-Bu) with vanadocene Cp2V yiel ds the heterodimetallic complexes Cp2V(mu-eta(2):eta(4)-butadiyne)Zr(C5H4R) (2) (4, R = t-Bu; 5, R = Me). Both compounds have been characterized by ele mental analysis, Raman spectroscopy, and magnetic susceptibility. According to the X-ray structure of Cp2V(mu-eta(2):eta(4)-butadiyne)Zr(C(5)H(4)t-Bu) (2) (4), the [Cp2V] and [(C(6)H(4)t-Bu)(2)Zr] metallocene moieties are bond ed-to a butadiene framework through the two internal carbon atoms for [Cp2V ] and through the two internal and the two external carbon atoms for [(C(5) H(4)t-Bu)(2)Zr]. The distances and angles observed around the internal carb on atoms of the butadiene framework indicate that both carbons are planar a nd tetracoordinate. These structural considerations are supported by a topo logical analysis of the electron localization function (ELF). The existence of the (C5H4R)(2)Zr(eta(4):1,2,3,4-PhC=C=C=CPh) (R = H, SiMe3, t-Bu, Me) z irconacyclocumulene species as a key intermediate in the formation of the ( Cp2V(mu-eta(2):eta(4)-butadiyne)Zr(C5H4R)(2) heterodimetallic compounds is evidenced by NMR studies of either the formation of such zirconacyclocumule ne species by addition to (C5H4R)(2)Zr(C=CPh)(2) of a catalytic amount of C p2V or of the formation of compounds 4 or 5 by addition of Cp2V to (C5H4R)( 2)Zr(eta(4):1,2,3,4-PhC=C=C=CPh) (R = t-Bu, Me, respectively; synthesized f rom dialkynyl zirconocene complexes (C5H4R)(2)Zr(C=CPh)(2) via h nu dayligh t). The X-ray structure of the (C5H4R)(2)Zr(C=CPh)(2) (R = H (6), t-Bu (7), SiMe3 (8)) dialkynyl zirconocene complexes is also reported.