Bis(propynyl)zirconocene (7a) reacts with [Cp(2)ZrCH(3)(THF)(+)[BPh(4)](-) (8a) to yield the [(mu-propynyl)(mu-eta(1):eta(2)-MeCCMe)(Cp(2)Zr)(2)] cation complex 5b that contains a planar-tetracoordinate carbon atom. Complex 5b was characterized by X-ray diffraction. The planar-tetracoordinate carbon center (C2) of 5b exhibits four strong bonds to its nearest in plane neighbors, i.e. to two carbon and two zirconium centers. In solution, complex 5b is dynamic. The C-s symmetric planar-tetracoordinate carbon compound 5b undergoes a thermally induced automerization reaction [Delta G(double dagger) (250 K) = 11.8 +/- 0.5 kcal mol(-1)] that proceeds through a C-2v-symmetric geometry (6b) that has the characteristics of a transition state according to a theoretical analysis of the 5 reversible arrow 6 reversible arrow 5’ rearrangement process. This means that the extra stabilization energy of planar-tetracoordinate carbon relative to ordinary planar-tricoordinate carbon in the "semi-fenestrane" environment of the complexes 5 amounts to ca 10-12 kcal mol(-1). The 5 reversible arrow 5’ equilibrium is strongly influenced by substituents at the mu-eta(1):eta(2)-RCCCH(3) ligand and by electronically active Cp substitution. In addition, a strong memory effect is operative in the formation of 5. It can be observed by using suitably substituted pairs of isomeric complexes 5. Thus, the reaction between [Cp(2)ZrCD(3)(THF)](+) (8d) and (MeCp)(2)-Zr(-C=C-CH3)(2) (7e) gives only complex 5h, which exhibits the CH3 group at the planar-tetracoordinate carbon center C2, whereas the reaction of the complementary reagent combination [(MeCp)(2)ZrCD3(THF)](+) (8e)/Cp(2)Zr-(-C=C-CH3)(2) (7a)] in this double-labeling experiment exclusively leads to the same type of product (5i) that has the CD3 group bonded to C2. This and other examples of this organometallic memory effect indicate that dimetallic intermediates are involved in the formation of the unusually structured stable planar-tetracoordinate carbon compounds 5.