PHOTOREDOX BEHAVIOR OF TRANS AND CIS-CO(III)-DIISOTHIOCYANATOTETRAM(M)INE COMPLEXES

The trans and cis isomers of Co(NCS)2(NH3)4+ and Co(NCS)2(en)2+ were p repared and characterized. Single-crystal X-ray diffraction showed tha t the first named was the trans isomer and established that the thiocy anates are N bonded. Photoredox quantum yields of the Co2+ ion on room temperature irradiation at 360 nm were found to be 0.065 +/- 0.001, 0 .082 +/- 0.001, 0.0088 +/- 0.0007 and 0.0040 +/- 0.0010 respectively. Other products measured were the thiocyanate ion and ammonia, but the relationships of these yields to Co2+ were not as expected for simple oxidation of thiocyanate and reduction of Co(III). On irradiation in t he presence of thiocyanate, a significant increase in yields occurs an d this can be modelled in terms of scavenging of the thiocyanate radic al from an initial caged radical pair giving picosecond estimates for the lifetime of this species of the order of tens of picoseconds, or i n terms of photolysis of a thiocyanate/complex ion pair giving formati on constants of 0.25 +/- 0.17, 0.12 +/- 0.04, 0.11 +/- 0.06 and 0.06 /- 0.04 for the complexes in the sequence above. In all but the last i nstance these are in excellent agreement with the values estimated by fitting the thiocyanate induced changes in the UV-visible spectra of t he complexes, 0.34 +/- 0.25, 0.14 +/- 0.07, 0.09 +/- 0.07 and 0.60 +/- 0.07 respectively. Laser flash photolysis studies on the nanosecond a nd picosecond time-scale failed to reveal any direct spectroscopic evi dence for a radical pair species, but the absorbance of (NCS)2-. was s een in all four systems. The effects of added electrolytes and of visc osity on the formation and decay of this intermediate were investigate d and are reported in detail.