The reaction of dimethylzirconocene with 2R,3R_diethyltartrate proceeds with evolution of two equivalents of methane and yields the chiral diethyltartratozirconocene dimer 2. The analogous reaction between CpzZr(CH,), and 2!?,3S_diethyltartrate furnishes ent-2 in almost quantitative yield. Complex 2 contains a dimetallatricyclic framework (tetraoxadizirconatricyclo[5.3.0.ti*6]decane) in solution as well as in the solid state. Complex 2 was characterized by X-ray diffraction. It crystallizes in space group P2, with cell parameters a =9.494(l), 6 = 20.122(2), c = 9.915(l) A, p= 10&36(l)", Z = 2, R = 0.039 and R, = 0.046. In solution the tartratozirconocene dimer 2 undergoes an intramolecular automerization reaction (AGti,,, (265 K) = 13 f 1 kcal mol-r) which probably proceeds via a ten-membered metallacyclic intermediate or transition state. A cross-over experiment between the enantiomeric complexes 2 and ent-2 showed the activation barrier for intermolecular exchange of mononuclear dioxazirconacyclopentane units Cp&rOCH(E)CH(E)Cl (E=CO&H,) leading to equilibration with the isomeric dinuclear tartratozirconocene complex meso-to be at AGSintcr (290 K) = 22 f 1 kcal mol-r. Complex meso-shows NMR spectra compatible with a static ten-membered dimetallamonocyclic ring structure. The reaction of dimethylzirconocene with meso-dimethyltartrate yields the (meso-dimethyltartrato)zirconocene dimer 4 (two isomers, 4a and 4b in a 60:40 ratio, both of which possess dynamic dimetallatricyclic structures in solution (AGSintra (224 K) = ll*l kcal mol-' (4a); AGSintra (285 K)= 14*1 kcal mol-' (4b); AG*intcr (300 K)=24.7&1.0 kcal mol-' (4a+4b). Crystallization from methylene chloride gives single crystals of the isomeric (mesodimethyltartrato)zirconocene dimer 5a which contains a monocyclic ten-membered ring structure. Complex 5a crystallizes in space group P2Jc with cell parameters a = 11.261(2), b = 14.511(2), c = 11.562(2) A, /3 = 117.81(l)"; Z=2, R=0.047 and R,=O.O63.