The desorption of conventional ligands from the surface of halide perovskite nanocrystals (NCs) often causes their structural instability and deterioration of the optoelectronic properties. To address this challenge, we present an approach of using a bidentate Lewis base ligand, namely, 1,4-bis(diphenylphosphino)butane (DBPP), for the synthesis of CsPbBr3 NCs. The phosphine group of DBPP has a strong interaction with the PbBr2 precursor, forming a highly crystalline intermediate complex during the reaction. In the presence of oleic acid, the uncoordinated phosphine group of DBPP is converted into the phosphonium cation, which strongly binds to the surface bromide of the formed CsPbBr3 NCs through hydrogen bonding. Density functional theory calculations suggest that DBPP can strongly bind to the undercoordinated lead and surface bromide ions of CsPbBr3 NCs through its unprotonated and protonated phosphine groups, respectively. The robust binding of DBPP to the surface of perovskite NCs helps to preserve their structural integrity under various environmental stresses. Moreover, the electron density and energy levels are regulated in DBPP-capped CsPbBr3 NCs by the donation of electrons from the ligands to the NCs, resulting in their improved photocatalytic CO2 reduction performance. Our study highlights the potential of using bidentate ligands to stabilize the surface of perovskite NCs and modulate their optical and electronic properties.