Two new ruthenium complexes [Ru(bipy)(2)(PDA)](2+) (1) and [Ru (phen)(2)(PDA)](2+) (2) (PDA = 1,10-phenanthroline-4,7-dicarboxaldehyde) have been synthesized to detect cyanide based on the well-known formation of cyanohydrins. Both 1[PF6](2) and 2[PF6](2) were fully characterized by various spectroscopic techniques and their solid state structures determined by single-crystal X-ray diffraction. Their anion binding properties in pure and aqueous acetonitrile were thoroughly examined using two different channels, i.e., UV-vis absorption and photoluminescence (PL). After addition of only 2 equiv of CN-, the PL intensity of 1[PF6](2) and 2[PF6](2) was enhanced similar to 55-fold within 15 s along with a diagnostic blue shift of the emission by more than 100 nm. PL titrations of 1[PF6](2) and 2[PF6](2) with CN- in CH3CN furnished the very high overall cyanohydrin formation constants log beta(-)([CN)(]) = 15.36 +/- 0.44 (beta(-)([CN)(]) = 23 x 10(15) M-2) and log beta(-)([CN)(]) = 16.37 +/- 0.53 (beta(-)([CN)(]) = 2.3 x 10(16) M-2), respectively. For both probes, the second constant, K-2, is about 57-84 times less than K-1, suggesting that the cyanohydrin reaction is stepwise. The stepwise mechanism is further supported by results of a H-1 NMR titration of 2[PF6](2) with CN-. The high selectivity of 2[PF6](2) for CN- was established by PL in the presence of other competing anions. Furthermore, the color change from orange-red to yellow and the appearance of a orange luminescence, which can be observed by the naked eye, provides a simple real-time method for cyanide detection. Finally, theoretical calculations were carried out to elucidate the details of the electronic structure and transitions involved in the ruthenium probes and their cyanide adducts.