A　3D　organic-inorganic　hybrid　compound,　(2-MepyH)3　[{Fe(1,10-phen)3}3][{Pr4Sb12O　18(OH)　Cl11.5}(TDC)4.5({Pr4Sb　12O18(OH)Cl9.5}　Cl)]×3　(2-Mepy)×28　H2O　(1;　2-Mepy=2-methylpyridine,　1,10-phen=1,10-phenanthroline,　H2TDC=thiophene-2,5-dicarboxylic　acid),　was　hydrothermally　synthesized　and　structurally　characterized.　Unusually,　two　kinds　of　high-nuclearity　clusters,　namely　[(Pr4Sb12O18　(OH)Cl11)(COO)5]5-　and　[(Pr4Sb　12O18　(OH)Cl9)Cl(COO)5]　4-,　coexist　in　the　structure　of　compound　1;　two　of　the　latter　clusters　are　doubly　bridged　by　two　μ2-Cl-　moieties　to　form　a　new　centrosymmetric　dimeric　cluster.　An　unprecedented　spontaneous　and　reversible　single-crystal-to-single-crystal　transformation　was　observed,　which　simultaneously　involved　a　notable　organic-ligand　movement　between　the　metal　ions　and　an　alteration　of　the　bridging　ion　in　the　dimeric　cluster,　induced　by　guest-release/re-adsorption,　thereby　giving　rise　to　the　interconversion　between　compound　1　and　the　compound　(2-MepyH)3[{Fe(1,10-phen)　3}3][{Pr4Sb12O18(OH)　Cl11.5}(TDC)4({Pr4Sb12O　18Cl10.5(TDC)0.5(H2O)　1.5}O0.5)]×　25　H2O　(1′).　The　mechanism　of　this　transformation　has　also　been　discussed　in　great　detail.　Photocatalytic　H2-evolution　activity　was　observed　for　compound　1′　under　UV　light　with　Pt　as　a　co-catalyst　and　MeOH　as　a　sacrificial　electron　donor.　Crystal　clear:　A　new　3D　organic-inorganic　hybrid　compound　that　contained　a　dimeric　Pr-Sb-oxochloride　cluster　exhibited　an　unprecedented　spontaneous　and　reversible　single-crystal-to-single-crystal　transformation　and　photocatalytic　H2-evolution　activity　(see　figure).　©　2013　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim.