We identify the reaction network governing gold monolayer protected cluster (MPC) formation during the reduction of Au(PPh3)Cl and L5 (L5 = 1,5-bis(diphenylphosphino)pentane) in solutions. UV-vis spectroscopy and electrospray ionization mass spectrometry (ESI-MS) monitored the formation of ligated Aux : 6 ≤ x ≤ 12 clusters, which comprise the reaction intermediates and final products. Initially, predominantly [Au2L52]2+ complexes form through dissolution of Au(PPh3)Cl. These complexes control the reduction and nucleation reactions that form nascent phosphine-ligated Au8 and Au10 ionic clusters. [Au10L54]2+ is an observed growth platform for ligated Au11 and Au12 clusters. The data for syntheses of Au:L5 systems evidence that the nascent reaction products (t 4); instead, after reduction ceases, subsequent solution phase processing provides greater control for tuning cluster nuclearity.