3-Hydroxyflavonate-diphenyltin(IV) chloride (Ph(2)Sn(Ofl)Cl) was prepared and characterized structurally by single crystal X-ray diffraction. The compound possesses high fluorescence (λ(max) 464 nm with excitation at 400 nm) in neutral aqueous solutions containing 5 mM cetyltrimethylammonium chloride and does not lose the flavonol ligand even at high dilution. The fluorescence is selectively quenched by pyrophosphate (PPi) (linear range 0-5 μM, detection limit 0.1 μM). Determination of 1 μM PPi tolerates the presence of 100-fold excess of AMP, ADP, inorganic phosphate and acetate, and 10-fold excess of ATP. From concentration profiles of the quenching effect it is concluded that quenching occurs due to formation of the complex (Ph(2)Sn(Ofl))(3)PPi, which is transformed at higher PPi concentrations to a simple 1 : 1 complex Ph(2)Sn(Ofl)(PPi). No flavonol ligand exchange with PPi is observed. A series of related organotin(IV) complexes were prepared in situ by reacting 3-hydroxyflavone with Me(2)SnCl(2), PhSnCl(3) and n-BuSnCl(3). The derivatives of Me(2)SnCl(2) and PhSnCl(3) behave similarly to Ph(2)Sn(Ofl)Cl but with lower sensitivity to PPi. However, the n-butyltin(IV) derivative tentatively of the composition n-BuSn(Ofl)(2) undergoes fluorescence enhancement rather than quenching in the presence of PPi in the μM concentration range. The results of this study demonstrate that the use of metal flavonol complexes as optical sensors for anions is a promising approach.