Polyoxometalate (POM) based coordination assemblies offer tunable platforms for modulating structural connectivity and photoinduced charge transfer, yet controlled switching of POM between coordinating linkers and non-coordinating counter-anions within a single system remains challenging. Herein, 14 POM-based lanthanide complexes (La-Lu, excluding Pm) were constructed using tetraethyl methylenediphosphonate as an auxiliary ligand. Single-crystal X-ray diffraction reveals a structural transition driven by lanthanide contraction: in complexes 1-4 (La-Nd), the POM coordinates directly to Ln³⁺ centers and bridges two [LnL₄]³⁺ units, whereas in complexes 5-14 (Sm-Lu), it acts as a counter-anion to discrete [LnL₄]³⁺ cations. This switching in coordination mode correlates with distinctly different optical and photocatalytic properties: complexes 1-4 exhibit narrower band gaps (2.94-2.95 eV) and visible-light activity, while complexes 5-14 show wider band gaps (3.22-3.27 eV) and respond mainly to UV light. Complex 1 achieves 94.68% ciprofloxacin degradation within 120 min under visible light (k = 0.02413 min^-1), with in situ XPS, EIS, band-structure analysis and  in situ EPR collectively supporting photoinduced charge redistribution, improved interfacial charge transport, and generation of •O₂^- and •OH. Meanwhile, complex 6 exhibits a Eu³⁺ centered luminescence lifetime of 3.34 ms and a quantum yield of 29.94%, suggesting radiative relaxation may compete with photocatalytic charge transfer. Collectively, these results support a correlation among lanthanide-contraction-induced coordination-mode transition, POM coordination behavior, and photocatalytic performance, providing a strategy for designing POM-based crystalline functional materials.