Equatorial vacancy

The reversible reaction of with complex 6 has been proposed to involve dihydrogen coordination to an equatorial position of the Ir(II) center, followed by single-metal addition and hydride migration steps. The axial-to-equatorial movement of the coordination vacancy required by this mechanism has been shown to be a facile process. 

The stereochemistry of the reaction products of Scheme 7 again suggests that these reactions involve single-metal additions preceded by dihydrogen coordination to equatorial positions. In this respect, it has been shown that complex 9 can readily dissociate its acetonitrile ligands, and exchange them from axial to equatorial positions with low kinetic barriers.[28] Such vacancy rearrangements... 

Filling anion vacancies with inserted fluorine anions completes the coordination polyhedron of the transition metal cation to an octahedron, which in the case of Jahn-Teller active cations leads to anisotropic coordination with four short equatorial and two longer apical metal-oxygen bonds. 

Figure 5.14 (a) Structure of the polyanion IP2W2 07j(OH2)3] and (b) arrangement of tungsten and oxygen atoms and water molecules near the equatorial plane of the anion. From (50] by permission. Structure of vacancy compounds (c) [P2W2o07o(OH2)2) and (d) [P2W2o072] - (e) Proposed structure for [P2W 9069(0H2)j . Reproduced by permission of Conseil National dc la Recherche du Canada from [40]... 

---