(Alkenyl)zirconocene chlorides Cp2Zr(CI)(CH=CHR) (4, R = H, phenyl, n-butyl, or cyclohexyl) react with "Cp2Zr" generated from various precursors to yield the neutral dinuclear alkenyl-bridged bis(metallocene) complexes 6 [Cp2Zr(mu-Cl)(mu-eta(1):eta(2)-CH=CHR)ZrCp2]. The complexes 6 are regioselectively protonated by treatment with (HNMe2Ph+)(BPh4-) (8a) to yield the unusually structured products 9 [Cp2Zr-(mu-Cl)(mu-eta(1):eta(2)-(CH2CHR)-H-1-H-2)ZrCp2+], which exhibit an unsymmetrically bridged hydrocarbyl ligand containing a novel type of a hypercoordinated carbon center (C-1) inside the rigid organometallic framework. From the NMR analysis and an X-ray crystal structure determination of the example 10c [MeCp)(2)Zr(mu-Cl)(mu-(CH2C2H)-H-1-n-butyl)Zr(MeCp)(2)(+)], it is evident that the hypercarbon atom C-1 is coordinated to four close neighboring atoms (Zr-1, Zr-2, C-2,and H-1a in a distorted square-planar arrangement with the remaining C-1-H-1b bond being oriented perpendicular to it. H-1b thus marks the apex of a distorted square pyramid; the hypercarbon atom C-1 is located in the center of the basal plane. This unusual structural coordination geometry around C-1 is determined by the stereoelectronic features of the two adjacent group 4 bent metallocene units. From the dynamic features of complex 9a (R=H), a stabilization energy of ca. 10 kcal mol(-1) is estimated for the uncommon coordination mode of C1, which is favored here, relative to a "normal" sp(3)-hybridized structure in a C-2v-symmetric metallacyclic framework. Part of the pronounced thermodynamic stabilization of this unusual pentacoordinate carbon geometry originates from the strong alpha-agostic Zr ... H-1a-C-1 interaction in the basal plane. From the monodeuterated derivative 9a-(CHD)-H-1, an energy difference of 220 cal mol(-1) between D and H favoring the bridging position was determined.