The reactions between dithizone (H(2)Dz, (1)) or potassium dithizonate (K(+)HDz(-), (2)) and phenylmercury(II) chloride gives PhHg(HDz), (3). Complex (3) is photochromic. In dichloromethane, the blue photo-exited state of (3) exhibits a first order relaxation process to regenerate the orange ground state with rate constant 0.00053 s(-1). The half life of this relaxation is ca. 1300 s. Electrochemically, on cyclic voltammetry time scale, the oxidations of (1) and (3) are different. A comparative voltammetric and spectro-electrochemical study of (1) and (3) in CH2Cl2 containing 0.1 mol dm(-3) [N(Bu-n)(4)][B(C6F5)(4)] revealed that the mercapto group of (1) can be oxidised in two one-electron transfer steps. A disulphide is first produced and then in a second oxidation step, HDz(+) is formed. In contrast, complex (3) shows only one ligand-based oxidation step. Upon complexation with phenyl mercury the free mercaptan group of (1) becomes a stable "metal thioether", Hg-S-C, which effectively prevents disulphide formation in (3) upon electrochemical oxidation. Both (1) and (3) shows two reduction steps. The electrochemical fingerprint of blue photo-excited (3) is identical to that of the orange ground state as no new functional groups are introduced upon irradiation: only bond rotation occurs. The different electronic spectra for each of the redox states of (3), obtained from spectro-electrochemical measurements, revealed that only the (3)/(3(-)) couple exhibits electrochromic properties. (C) 2011 Elsevier Ltd. All rights reserved.