We report the electrochemical activity and the mechanism of formation of a mixed valence polyoxometalate-based organic hybrid cluster with the formula [Na(SO3)2(PhPO3)4MoV4MoVI14O49]5- (1). Electrochemical investigations of the mixed valence compound 1 showed three redox couples, in which the electrons were mainly delocalized over eight Mo sites. Furthermore, the synthesis was investigated using 31P-NMR, which showed that the self-assembly of cluster 1 was triggered by the addition of organic solvents, and was largely independent of the nature of the solvents, suggesting that a decrease in the concentration of water promoted cluster assembly. Finally the stability of 1 was explored and we concluded that the use of phenylphosphonate allowed the covalent stabilization of the [MoV4MoVI14] core.