Transition metal hydroxides, comprising metal-centered polyhedra, are abundant in nature and have a broad range of applications. Although the intercalation/deintercalation of polyhedra with unconventional coordination numbers (UCN) plays a pivotal role in their formation process and influences their chemical behavior, the detailed mechanism remains obscure. Here, taking the kinetically controlled growth of Co(OH)₂ as a model system (where polyhedra with UCN refers to 4-coordinated tetrahedra), by combining in situ pH measurement and in situ ultraviolet-visible (UV-vis) spectroscopy, we tracked Co²⁺ tetrahedral intercalation/deintercalation with varying OH- concentrations, discovering that the initial Co²⁺ tetrahedral intercalation into the Co(OH)₂ lattice is inevitable and its retention is influenced by the effective OH- concentration, challenging previous beliefs about the formation of Co(OH)₂ and potentially other hydroxides. More importantly, an understanding of the intercalation mechanism would significantly contribute to optimization of the synthesis conditions of hydroxides with tunable coordination environments, which hold significant application potential, as evidenced by a proof-of-concept application in the oxygen evolution reaction.