LOW TEMPERATURE STUDIES OF MIXED VALENCE DINUCLEAR Mn COMPOUNDS. Claire Wilson, F. Krebs Larsen, Department of Inorganic Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark, and Brian N. Figgis, Department of Chemistry, University of Western Australia, Nedlands, WA 6009, Australia.

Mixed valence Mn compounds provide important, small molecule models for the metalloenzyme OEC (Oxygen Evolving Complex) of photosystem II involved in photosynthesis in green plants.

The first charge density study of a bis(u-oxo)Mn(III)Mn(IV) dimer, (bis(u-oxo) tetrakis (2,2'-bipyridine) dimanganese(III,IV),¹ revealed a substantial accumulation of electron density at the u-oxygen atoms in the MnO₂ ring. In an attempt to follow up this study with a combined X-ray and neutron diffraction study of the perchlorate salt of this compound different media for crystal growth were tested, particularly with the growth of large crystals for the neutron study in mind.

Very nice crystals were indeed formed with the surprising incorporation of nitrobenzene molecules in the crystal. X-ray data for these crystals have been collected at room temperature, 200 K and 9 K. However, some effect causing reversible splitting of diffraction intensities is at work below 200 K resulting in the 200 K structure being the best determined at this point.

Crystal data: Space group P2_l/n (No. 14). a= 16.374(6), b= 15.515(7), c= 21.969(8) Å, ß = 93.17(2)deg. at 200 K.

1. Anette Frost Jensen, Zhengwei Su, Niels K. Hansen and Finn Krebs Larsen. Inorg. Chem. 34 (1995) 4244-4252.