Organozinc halide reagents are not as simple as RZnCl. Differences in solution NMR spectra and kinetic behavior of phenyzinc halide reagents prepared from Grignard (1a system) and lithium reagents (1b system), respectively, were observed. In the 1a system, zero-order kinetic behavior indicates that transmetalation is not the rate-limiting step, while first-order kinetics were observed in the 1b system, and the transmetalation step proved to be rate-limiting. The structure of 1a was revealed by single-crystal X-ray diffraction analysis to contain (THF)(4)MgCl(2) complexed to PhZnX. DFT calculations suggest weaker Ph-Zn bonds in 1a than in 1b, consistent with the faster transmetalation process in catalytic reactions.