In complexes of the type [(NC)2HgIINCFeII(CN)5]4- which are formed in aqueous mixtures of Hg(CN)2 and [Fe(CN)6]4- both metals are weakly coupled. Optical metal to metal charge transfer (MMCT) excitation from FeII to HgII leads to the reduction of HgII to Hg0 and the oxidation of
[Fe(CN)6]4- to [Fe(CN)6]3-. In the complex [(NC)5CoI-HgII-CoI(CN)5]6- the metals are strongly coupled by direct but polar metal metal bonds. MMCT (CoI -> HgII) excitation induces the photoredox reaction: [HgCo2(CN)10]6- -> Hg0 + 2[Co(CN)5]3-.