It is unclear at the molecular level why HY (HY = RSH, or ROH, or RNH₂) with HPPh₂ additives kinetically affects the reaction pathway to the formation of different monomers (Ph₂P–SeCd–Y or Ph₂P–SeCdSe–Y) in the systhesis of semiconductor nanocrystals. In the present work, it was found that in a [Cd(OA)₂ + SeP(C₈H₁₇)₃ + HPPh₂ + HY] mixture, HY behaves as a mediator for the formation of the initial kind of monomer, besides as a hydrogen/proton donor in the release of oleic acid and as an accelerant in the Se–P bond cleavage, which follows the mechanism of hydrogen-shift/nucleophilic-attack. The capability of the HY additive to provide a H-source decreases in the order SePPh₂H > RSH > HPPh₂ > ROH > RNH₂, while the performance of HY to accelerate Se–P bond cleavage decreases in the order HPPh₂ > RSH > RNH₂ > ROH. The capacity of HY to promote the formation of the Ph₂P–SeCd–Y monomer decreases in the order RSH > HPPh₂ > ROH > RNH₂, while the effect of HY to drive the formation of the Ph₂P–SeCdSe–Y monomer decreases in the order HPPh₂ > RSH > RNH₂ > ROH. The activation strain energy plays a key role in both the Se–P and H–Y bond cleavage, which correlates negatively to the size of the coordinated atom radius. When only HPPh₂ is present without other HY species (HY = RNH₂, or RSH, or ROH), Ph₂P–SeCdSe–PPh₂ is preferentially formed. Alternatively, when both HY (HY = RNH₂, or RSH, or ROH) and HPPh₂ are present, Ph₂P–SeCd–Y is favorably formed. For the formation of Ph₂P–SeCd–Y (Y = –PPh₂, –SR, –OR, and –NHR), SePPh₂H embodies the catalytic performance, while HPPh₂ serves as the catalyst for the formation of Ph₂P–SeCdSe–Y (Y = –NHR or –OR). Our study brings a molecular-level insight into the relationship between the CdSe monomer and the phosphorous-containing side-product, which may advance the rational design and synthesis of quantum dots.