Abstract <p>Manganese(II) and copper(II) complexes of tripodal tris[2-(2'-pyridylmethoxy)phenyl]phosphine oxide (L) have been synthesized and characterized by IR spectra and X-ray diffraction as powders and crystals and in solutions. In crystalline and powder state of complex [MnLCl₂], the ligand shows bidentate O,N-coordination. Reaction of ligand L with Cu(NO₃)₂·3H₂O results in complexes of composition M : L = 1 : 1 and 1 : 2. In crystalline monoligand complex [CuL(NO₃)]⁺(NO₃)^–, phosphine oxide behaves as a bridge-chelating ligand and binds metal ion by the oxygen atom of the PO group, the ethereal oxygen atom, and the nitrogen atom of one of the Py fragments. In the bis-ligand powder complex, coordination polyhedron is formed by the phosphoryl groups and the nitrogen atoms of one of the Py fragments of two ligands, and a nitrate ion ([Cu(O_PO,N–L)₂NO₃]⁺NO<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(_{3}^{ - }\)</EquationSource> <!--InrgChem2560263Baulina-m1--> </InlineEquation>). In solutions, coordination polyhedra of the complexes are labile and are dependent on solvent. The copper complexes are efficient catalysts for the decomposition of Methylene Blue by the Fenton reaction upon exposure to monochromatic light with λ = 365 nm.</p>