A novel PtIV azido triazolato complex exists as an equilibrium between two species in d3-MeCN and evolves azide radicals (but not hydroxide radicals) when irradiated with visible light.

Reaction between the platinum(IV) azido complex trans,trans , trans ‐[Pt(py) 2 (N 3 ) 2 (OH) 2 ] ( 1 ) and 1,4‐diphenyl‐2‐butyne‐1,4‐dione 2 in MeCN produces the intermediate peroxide‐bridged dimeric platinum(IV) azido triazolato species ( 5 ), which has been characterised by X‐ray crystallography. However, if the reaction between 1 and 2 is conducted in MeOH it results in decomposition. Over time in MeCN, dimer ( 5 ) converts into mononuclear complexes trans,trans , trans ‐[Pt(py) 2 (N 3 )(triazole)(OH) 2 ] ( 3 a / 3 b ), which are in dynamic exchange. If resuspended in protic solvents (MeOH,H 2 O), 3 a / 3 b undergo a slow (22 d) irreversible rearrangement to a cyclised platinum(IV) species 4 which contains a formally N,O‐chelated ligand. Conversion of 3 a / 3 b to 4 in d 4 ‐MeOH can be accelerated (384x) by irradiation with visible light, although continued irradiation also produces N 3 . and OH . radicals, and the [ 4 ‐N 3 ] + species can be readily detected by ESI‐MS. Solvent choice significantly effects both the cycloaddition reaction between 1 and 2 , and the stability of the resultant complexes.