Heterodentate phosphines containing anionic organophosphorus groups remain virtually unexplored ligands in the coordination chemistry of coinage metals. A hybrid phosphine–phosphine oxide (o-Ph2PC6H4)2P­(O)H ( HP 3 O ) readily forms the disilver complex [Ag2( P 3 O )2] (1) upon deprotonation of the (O)­P–H fragment. Due to the electron-rich nature, the anionic phosphide oxide unit in 1 takes part in efficient intermolecular hydrogen bonding, which has an unusual and remarkably strong impact on the photoluminescence of 1, changing the emission from red (644 nm) to green-yellow (539 nm) in the solid. The basicity of the R2(O)­P– group and its affinity for both inter- and intramolecular donor–acceptor interactions allow converting 1 into hydrohalogenated (2, 3) and boronated (4) derivatives, which reveal a gradual hypsochromic shift of luminescence, reaching the wavelength of 489 nm. Systematic variable-temperature analysis of the excited state properties suggests that thermally activated delayed fluorescence is involved in the emission process. The long-lived excited states for 1–4, the energy of which is largely regulated by means of the phosphide oxide unit, are potentially suitable for triplet energy transfer photocatalysis. With the highest T1 energy among 1–4, complex 4 demonstrates excellent photocatalytic activity in a [2+2] cycloaddition reaction, which has been realized for the first time for silver­(I) compounds.