Photoredox behaviour 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 prepared and characterized. Single-crystal X-ray diffraction showed that the first named was the trans isomer and established that the thiocyanates 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, 0.0082±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 irradation in the presence of thiocyanate, a significant increase in yields occurs and this can be modelled in terms of scavenging of the thiocyanate radical from an initial caged radical pair giving picosecond estimates for the lifetime of this species of the order of tens of picoseconds, or in terms of photolysis of a thiocyanate/complex ion pair giving formation constant 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 instance these are in excellent agreement with the values estimated by fitting the thiocynate induced changes in the UV-visible spectra of the 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 and picosecond time-scale failed to reveal any direct spectroscopic evidence for a radical pair species, but the absorbance of (NCS)₂^-√ was seen in all four systems. The effects of added electrolytes and of viscosity on the formation and decay of this intermediate were investigated and are reported in detail.