Organometallic compounds of type [M(3)(CO)(7)(dppm)(tolan)], where M is Os or Ru, dppm is bis(diphenylphosphino)methane and tolan is 1,2-diphenyl-acetylene, have two isomers differing in the orientation of alkyne ligand with respect to the triangular metallic frame. The perpendicular isomer is stable in the oxidized state while the parallel isomer exists in the reduced state. The redox-induced isomerization causes two unusual adsorption waves to be observed on mercury in acetone solution. The osmium cluster gives two adsorption post-waves, whereas the ruthenium cluster gives one adsorption pre-wave and one post-wave. These effects are explained in terms of the different location of the isomerization reaction in the two-electron redox exchange.