Density functional theory (DFT) calculations have been performed to investigate the origin of the reactivity and ligand-transfer selectivity of organozincates in the 1,2-addition to carbonyl compounds. Examination of the addition of Me3ZnLi to formaldehyde as compared with that of Me2Zn showed that the addition reaction is facilitated by the push-pull synergy of the Lewis acidic Li atom and the negatively charged Me3Zn moiety. This analysis then provided an answer to the mechanistic question about the experimentally established ligand-transfer selectivity in the 1,2-addition of heteroleptic organozincate Me2Zn(X)Li (X = H-, R2N-, and R3Si-). The addition of these heteroleptic zincate compounds results in selective transfer of H, R2N, and R3Si groups owing to the favorable orbital interaction between these groups and the carbonyl pi-system. The addition reaction of the zincate compounds conforms to the mechanistic framework of the conventional nucleophilic reaction, such as the addition reaction of MeLi dimer, and is different from the reaction of organocuprates, where oxidation/reduction of the copper atom is involved.