Both diastereoisomeric forms of [Ru-2(dppcb)(biPY)(4)](4+), where dppcb is cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane, show a completely unexpected photochemical reactivity, In acetonitrile, a ligand exchange reaction produces [Ru-2(dppcb)(biPY)(2)(MeCN)(4)](4+). To the best of our knowledge, dppcb is the first phosphine leading to this reaction type. A careful examination of the X-ray structures of the diastereoisomeric ally pure forms of [Ru-2(dppcb)(bipy)(4)](4+) and of [Ru-2(dppcb)(bipy)(2)(MeCN)(4)](4+) reveals that the release of steric strain observable as the disappearance of mechanical coupling is responsible for this reactivity. [GRAPHICS]