Inovganic Chemistry Laboratories, Irn9eria2 College, London SW7 2AY) Summury Equimolar mixtures of Ln(fod), (fod = C,F,COCHCOCMe,) and C,F,CO,Ag act as shift reagents for alkenes in CCI, or CDC1, solution.

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Uranium( IV) -Crown-etherComplexes and Cryptates. 'H Nuclear Magnetic Resonance Study, and X-Ray Crystal and Molecular Structure of Tetrachloro -(2,5,8,15,18,2 1hexaoxatr icyclo [20.4.8 .09i14] hexacosane)uranium( IV)

A t low temperatures, ring inversion is slow on a proton n.m.r. time-scale for the Pr( [2Ho]fod),-NNN'N'tetramethylethylenediamine { [2Hg]fod = (CD,),CC( : 0)-CHC( :O)CF,CF,CF,) complex (I), and the barrier to ring inversion, Act, is 42-2 kJ mol-1 a t -38 O C; in the corresponding NNN'N'-tetramethyl-1,8-propylenediamine complex (11), interchange of N-Me groups is observed above room temperature.

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