Abstract The photochemistry and thermal stability of [Cu(HB(3,5-Me 2pz) 3)] 2O ( 1) and [Cu(HB(3,5-Me 2pz) 3)] 2CO 3 ( 2) have been investigated in various solvents in view of their utilisation as catalysts in olefin functionalization reactions. Irradiation of the above complexes in chlorinated aliphatic solvents within the CT-band region generates the mononuclear complex [Cu(HB(3,5-Me 2pz) 3)Cl] ( 4), which exists as a tetracoordinated monomer in solution at room temperature, and as a dimer (pentacoordinated-Cu(II) species, 4″) in a frozen solution or in the solid state. In coordinating solvents 4 is converted into the monomer pentacoordinated complex [Cu(HB(3,5-Me 2pz) 3)Cl(Solv)] ( 4′). The photo-generation of 4 implies the homolytic cleavage of the Cu–O bond in 1, with formation of the reactive radical species [LCu(I)] and [LCu(II) O . ] [L = (HB(3,5-Me 2pz) 3)]. 2 appears to be more stable than 1, under the same conditions. The radical Cu(II)-species [LCu(II) O . ], according to the solvent and conditions, can either be reduced to [LCu(I)], or undergo an intramolecular rearrangement.