Tailoring precursor reactivity in colloidal nanocrystals (NCs) is a powerful strategy to achieve the uniformity. The gold precursor reactivity was engineered in multistep cation exchange reactions of Ag-Au-S quantum dots (QDs) through ligand coordination, metal-metal bonding, and steric effects, resulting in a series of discrete exchange pathways. With the conventional precursor HAuCl₄, Ag₂S QDs undergo complete conversion to Au₂S QDs via Ag₃AuS₂ and AgAuS alloy QD intermediates; however, this process yields compositionally inhomogeneous QDs or QD mixtures of multiple phases, producing broad or bimodal photoluminescence (PL). In contrast, the mononuclear phosphine-coordinated precursor AuPPh₃Cl selectively halts exchange at AgAuS QDs even in large excess, affording phase-pure Ag₃AuS₂ and AgAuS QDs with brighter and narrower PL. Further suppression of reactivity was achieved with multinuclear AgAu<i>_m</i>(PPh₃)_<i>n</i>Cl_<i>m</i>+1 complexes, which stalled exchange precisely at Ag₃AuS₂ QDs as phase-pure. Mechanistic analyses using ³¹P NMR and mass spectrometry revealed that AuPPh₃Cl undergoes in situ transformation into multinuclear Ag-Au-phosphine complexes that attenuate exchange through Ag-Au interactions and steric hindrance. This reactivity-controlled exchange enables precise phase targeting across a QD size series while preserving the size and morphology, yielding narrow and tunable emission spanning 1.04-1.87 eV (663-1192 nm) in the red-to-near-infrared spectral region. These findings establish precursor reactivity engineering as a powerful design principle for achieving phase-pure alloyed NCs and broaden opportunities for optoelectronic devices, infrared bioimaging, and other applications requiring spectrally precise nanomaterials.