text: The crystal structures of the THF complexes of the Reformatsky reagents tert-butyl bromozincacetate and  ethyl bromozincacetate have been determined. Both form cyclic eight-membered dimers in the solid state, but differ in stereochemistry: the eight-membered ring in the ethyl derivative adopts a tub-shaped conformation and has cis bromo groups and cis THF ligands, whereas in the tert-butyl derivative, the ring is in a chair form and the bromo groups and THF ligands are trans.  Note that, in contrast to lithium and boron enolates, which have the metal(loid)s exclusively bond to oxygen, the zinc enolate moiety in the Reformatsky reagents have zinc atoms that are simultaneously O- and C-bound and can therefore be described as "organometallic".