Growth of monodisperse InP quantum dots (QDs) is a pressing requirement in display applications, as the size uniformity is related to color purity in display products. This study reports the colloidal synthesis of InP QDs in the presence of Zn precursors, in which size uniformity is markedly enhanced as compared to the case of InP QDs synthesized without Zn precursors. The nuclear magnetic resonance spectroscopy and mass spectrometry analyses on aliquots taken during the synthesis allowed monitoring the appearance of metal-phosphorus complex intermediates in the growth of InP QDs. In the presence of zinc carboxylate, intermediate species containing Zn-P bonding appears. The Zn-P intermediate complex with $P(SiMe_3)_3$ exhibits lower reactivity than In-P complex, which is corroborated by predictions based on density functional theory and electrostatic potential charge analysis. The formation of stable Zn-P intermediate complex results in lower reactivity, hence monodisperse QDs. Insights from the experimental and theoretical studies advance the mechanistic understanding and controlling of nucleation and growth of InP QDs, key to the preparation of monodisperse InP-based QDs in meeting the demand of display market.