Precursor compounds (PCs) link quantum dots (QDs) and magic-sized clusters (MSCs), which is pivotal in the conversion between QDs and MSCs. Here, for the first time, we report the transformation, synthesis, and composition of a type of ZnSe PCs. ZnSe PCs can be directly transformed to two different MSCs with the assistance of octylamine and acetic acid at room temperature. The two types of MSCs exhibit sharp absorption peaks at 299 and 328 nm which are denoted as MSC-299 and MSC-328. In the preparation of ZnSe PCs, diphenylphosphine (DPP) as an additive plays a key role which not only inhibits the thermal decomposition of Zn precursor, but also acts as a reducing agent to reduce the by-products produced in the reaction. The composition was explored by X-ray photoelectron spectroscopy, energy dispersive spectrometer, matrix-assisted laser desorption/ionization time-of-flight mass spectra with ZnSe PC powder appeared as white powder after purifying by toluene (Tol) and methanol (MeOH). The results indicate that the molar ratio of Zn/Se is 2:1 with a molecular of ∼ 3, 350 Da. Therefore, we propose that the molecular formula of ZnSe PCs is Zn₃₂Se₁₆. In addition, at the molecular level, the covalent bond of Zn–Se is formed in ZnSe PCs. This study offers a deeper understanding of the transformation from PCs to MSCs and for the first time proposes the composition of PCs. Meanwhile, this research provides us with a new understanding of the role of DPP in the synthesis of colloidal semiconductor nanoparticles.