Cyclic voltammetry measurements show an unprecedented solvent dependence of metal-metal coupling for the symmetric mixed valence [3,2] complex, [{(NH 3) 5Ru} 2(μ-dicyd)] 3+, where dicyd 2− is the 1,4-dicyanamidobenzene dianion. The comproportionation constant was determined to be K c=10 and 68 400 in aqueous and acetonitrile solution, respectively. The intervalence absorption band was deconvoluted from the low energy Ru(III)-cyanamide LMCT band by curve fitting analysis which gave for the [3,2] complex assuming a gaussian IT band, E op=8190 cm −1, Δ ν 1 2 =4090 cm −1 and ϵ max=2590 M −1 cm −1 in aqueous solution and in acetonitrile solution E op=6910 cm −1, Δ ν 1 2 =2640 cm −1 and ϵ max=19 400 M −1 cm −1. It is suggested that donor-acceptor interactions between the solvent and ammine protons weakens the Ru(III)-cyanamide π bond. Because the interaction between Ru(III) and the cyanamide group is crucial to superexchange, metal-metal coupling is also weakened. The acceptor properties of water may also play a role.