The crystal structures of MoO(O2)zPO[N(CH3)213H20 (A) and MoO(O2)2PO[N(CH3)2]3CsHsN (B) have been determined. Three-dimensional X-ray data were collected on a linear diffractometer (Mo K~ radiation). The structures were solved by Patterson methods and refined by full-matrix least-squares analysis to a reliability index R= 0.032 for (A) and R= 0.048 for (B). In each compound the molybdenum atoms are surrounded by seven atoms forming a pentagonal bipyramid. The equatorial plane of the bipyramid contains the two peroxo groups and the oxygen atom of the phosphoramide. The apical positions are occupied by a double-bonded oxygen atom and a water molecule for (A) and a pyridine molecule for (B).

The crystal structures of dipyridinium p-oxo-tetraperoxo-dioxo-diaquo-dimolybdate(VI) 2 pyH + O[MoO(O2)2H20] 2-(A) and dipyridinium di-p-hydroperoxo-tetraperoxo-dioxo-dimolybdate(VI) 2 pyH ÷ [MoO(Oz)2OOH] z-(B) have been determined by X-ray analysis. Three-dimensional X-ray data were collected on a linear diffractometer (Mo Ke radiation). The structures were solved by Patterson methods and refined by full-matrix least-squares analysis to a reliability index R=0.035 for (A) and R=0.032 for (B). In each compound the molybdenum atoms are surrounded by seven oxygen atoms in a pentagonal bipyramidal arrangement. In (A) the anion consists of two pentagonal bipyramids joined by one corner, in (B) the anion consists of two bipyramids sharing one edge.

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An acidity function has been determined spectrophotometrically in aqueous sulphuric acid, at 298 K, by using seven secondary amine indicators. This function is the same as that based on primary amines and so differs from that relative to tertiary amines. Therefore, the first two types of amines are the indicators of the same acidity function. It is concluded that for a given acidity function, the Hammett postulate is only verified for all thecompounds characterized by the same value of a r structural parameter: apparent ratio of solvation.

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