Recent advances in the study of Aurivillius phases (Bi2A n -1BnO3n+3, n = 1−5) as oxide ion conductors are presented. The structures of modified Aurivillius phases containing extrinsic oxygen vacancies as well as Aurivillius phases containing intrinsic oxygen vacancies are surveyed. A detailed discussion of the conductivity behavior of these Aurivillius phases is given.

No abstract

Neopentylidene complexes of the type W(CH-t-Bu)(NAr)(OR), (Ar = 2,6-C6H3-i-Pr2; OR = 0-t-Bu, OCMe2(CF3), OAr) react with ethylene to give trigonal-bipyramidal or square-pyramidal tungstacyclobutane complexes. A square-pyramidal form is observed when OR = 0-t-Bu. Both forms are present when OR = OCMe2(CFJ or OAr, and they interconvert at a rate that is on the order of the NMR time scale. A complex of the type W[CH,CH(R)CH,](NAr)(OAr), is a square pyramid when R = t-Bu but a trigonal bipyramid when R = SiMe3. Square-pyramidal tungstacycles are characterized by comparable chemical shifts for a-and @-protons, chemical shifts for a-and @-carbon atoms that differ by only -25 ppm, and aliphatic JcH values, as opposed to large differences in chemical shifts between a-and @-carbons and a-and @-hydrogens and olefinic JCH values in TBP tungstacycles. Unsubstituted metallacycles react with excesshas been characterized crystallographical 2 = 8, p = 40.05 cm-', R = 0.056, R, = 0.082). The most characteristic feature of the WC, ring in the SP complex is the relatively normal W---C, distance of -2.8 A and W-C, bond lengths of 2.15-2.20 A, as opposed to a short W---C, distance of -2.4 8, and W-C, bond lengths of 2.05-2.10 A in TBP complexes. It is proposed that square-pyramidal complexes form in order to avoid competition between a relatively basic axial alkoxide and an axial imido ligand in a trigonal bipyramid. The reaction between W[CH,CH(t-Bu)CH,](NAr)(OAr), and ethylene to give W[CH2CH2CH2] (NAr)(OAr), and tert-butylethylene is zero order in ethylene and first order in tungsten between 9 and 34 "C with AH* = 19.7 kcal mol-' and AS* = -6 eu. Reactions between ethylene and trigonal-bipyramidal W[CH,CH(Me,Si)CH,](NAr)(OAr),, W[CH2CH(Me3Si)CH2](NAr)-[OCMe(CF3),I2, and W[CH2CH(Me3Si)CH2] (NAr) [OC(CF3)2(CF2CF2CF3)]2 are analogous but have positive values for AS' (11-23 eu). It is proposed that square-pyramidal tert-butoxide complexes are relatively stable toward loss of an olefin because the WC3 ring is further from an "olefin/alkylidene" transition state than is the WC3 ring in a trigonal bipyramid and in general for that reason complexes that contain relatively electron-withdrawing alkoxides will lose an olefin more readily than those that contain relatively electron-donating alkoxides.(space group P4,/n, a = 25.26 (2) A, c = 9.720 (5) A, V = 6202 A3, M, = 711.48, p(ca1cd) = 1.524 g cm-P ,

No abstract