The synthesis, structural characterization, and property evaluation of novel Manganese(II) and Iron(II) metal complexes using polyphosphonic acid ligands are presented in this study. Crystallographic bulk substances and individual crystals suitable for XRD were effectively isolated via benchtop solution precipitation and Teflon-lined hydrothermal methods, respectively. The synthesis of the Iron(II) derivatives was carried out under a strict anaerobic nitrogen atmosphere to prevent the atmospheric oxidation of the phosphonate arms without causing metal-hydroxide precipitation. Complexation was verified by FT-IR spectroscopy, which revealed modifications in the ν(N-H) and ν (O-H) regions as well as a distinctive shift of the v(P=O) stretching bands to lower wavenumbers (30–50 cm⁻¹). TGA made a distinction between directly coordinated water molecules (stable up to 150–250°C) and weakly bound lattice water molecules (lost at 50–120°C). According to magnetic susceptibility measurements, both metal centers exhibit collective ferromagnetic exchange interactions at low temperatures mediated by the bridging phosphonate oxygen pathways, adopting a high-spin configuration at room temperature (μeff ~5.92μB for Mn²⁺ and ~4.90μB for Fe2+). Additionally, structural investigation showed that large, dense supramolecular hydrogen-bonding networks are formed by uncoordinated phosphonate hydroxyl and amine groups. The promise of these complexes as functional materials for solid-state proton conductivity and molecular magnetic applications is highlighted by their extended networks, which solidify the crystal matrices into 2D layers or 3D frameworks.