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NIST Crystal Data developed at The National Institute for Standards and Technology has been incorporated with Siemens single crystal software for data collection on four-circle and two-dimensional CCD diffractometers. Why this database is useful in the process of single crystal structure determination, and how the database is searched, are described. Ideas for future access to this and other databases are presented.

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The reaction of Ph2P(NSiMe3)[N(SiMe3)2] with RSeCI3 (R = Ph, Me, Et) in acetonitrile produces 1 ,5-Ph4P2N4Se2R2 [R = Ph, ( l a ) ; Me, ( l b ) ; Et, (lc)] and l,5-Ph4P2N4Se2, (2a) when R = M e or Et; X-ray crystallography shows that ( l b ) is an eight-membered chair with the two selenium atoms displaced o n either side of the P2N4 plane b y 1.073(2) A.The preparation, molecular and electronic structures, and reactivity of cyclophosphathiazenes containing two-co-ordinate sulphur, (R2PN),(SN)2 ( x = 1,2,4),1,2 or three-co-ordinate sulphur, (R2PN),(NSX),,1J have been investigated extensively, but the corresponding cyclophosphaselenazenes are unknown. We report here the synthesis of the eightmembered rings (la-) by the cyclocondensation reaction of RSeC13 (R = Ph, Me, Et) with Ph~P(NsiMe3)[N(SiMe~)~l and the X-ray structure of (lb). When R = Me or Et this reaction also produces (2a), and 31P NMR data for (2a) and the