Four-site cooperative spin crossover (SCO) in a mononuclear Fe(II) complex has been observed for the isoselenocyanate complex [(bpte)Fe II (NCSe) 2 ] (bpte = S,S'-bis(2-pyridylmethyl)-1,2-thioethane). The compound has been characterised in four polymorphic crystalline phases. One is SCO-inactive (b phase), one exhibits one-step SCO (g phase) and two exhibit four-site cooperative SCO (a and d phases) as a function of temperature. Crystal structures have been determined over the temperature range 300-100 K for all four polymorphs, and at intervals of 5 K for the most interesting a and d phases. Of the five potentially addressable local spin configurations in a four-site cooperative system, four are observed as ordered crystal structures amongst the polymorphs. The d phase exhibits a remarkable ordered 1HS:3LS structure (HS = high-spin Fe(II), LS = low-spin Fe(II)) below ca 130 K, which establishes conclusively the four-site cooperative nature of the system. The a and d phases are polytypes, but it is difficult to link conclusively the associated two-dimensional structural similarity with the four-site cooperativity. Keywords: spin crossover; polymorphism; magnetism MS27-O4 Synthesis and crystal structure of few new decavanadates. Wieslaw £asocha ab , Vanadates and peroxovanadates are used as valuable catalysts. They can be also applied in medicine thanks to their enzymo-imitative properties. During our studies of new peroxo-vanadates and poli-vanadates we have synthesized three new compounds which belong to the group of deca-vanadates: two organic decavanadates and a rubidium decavanadate. The obtained compounds have the same, island-like type of anion, which is built of VO 6 octahedra. The structure of deca-vanadates can be described on the basis of the structures of b-octamolybdates. If in a-octamolybdate anion, Mo(VI) ions are replaced by V(V) ions V 8 O 26-12 ions will be obtained. When to the V 8 O 26 12 ion two VO 6 octahedra (or VO 3+ after removal of the redundant O atoms) are attached V 10 O 28 6-anion is created. The syntheses were carried out in 30% H 2 O 2. After crystallization, which lasted several days, deca-vanadates without O 2-2 groups were obtained. Explanation of this phenomenon will be the subject of our further research. Lattice parameters and chemical formulas of the obtained compounds: 1. Rb 6 V 10 O 28 .8H 2 O: P2 1 /c, 13.974, 10.595, 34.291 Å, 90, 97.437, 90°, V= 5034.23 Å 3 , Z=6. 2. 5(1-ammoniumpropane)(H 3 O +)V 10 O 28 .H 2 O, P-1, 14.507, 15.502, 15.858Å, 88.22, 81.53, 83.94°, V=3507.21Å 3. Z=3, 3. 2(1,2-diammoniumpropane).2(H 3 O +)V 10 O 28 .4H 2 O, P-1, 8.636, 10.394, 10.959Å, 68.86, 78.92, 66.99°, V= 843.Å 3 .Z=1. Fig. 1. Asymmetric unit of compound 3. Properties and geometry of the synthesized compounds will be compared with the properties and structures of several decavanadates described in a comprehensive survey recently published [1]. The investigated compounds form complex structures often with several molecules in an asymmetric unit. For example, in the structure of the de...