Pentagonal-Bipyramid Structure

As for the previous oxidation state, the chemistries of Mo" and W" are much more extensive than that of Cr" whieh is largely confined to peroxo- and fiuoro- complexes. [Cr(02)2(NH3)3], which has a dark red-brown metallic lustre, may be obtained either by treating [Cr(02)4] " with warm aqueous ammonia or by the action of H2O2 on ammoniacal solutions of (NH4)2Cr04. It has a pentagonal bipyramidal structure in which the peroxo- groups oeeupy... 

Figure 11 Character of the arrangement of the spin magnetic moments, and average magnetic moment, in seven-atom clusters with a pentagonal bipyramid structure and interatomic distances ranging from <4ulk to 80% <4ulk- Reproduced with permission from Ref. 79.

Let US consider the repulsive force model. The repulsive force is proportional to the inverse power n) of the distance (r) force l/r . Based on this consideration alone the pentagonal bipyramidal structure seems to be more stable for small values of n, the capped trigonal prism for intermediate values and the capped octahedron for large values of n, upto the limit of hard-sphere model. It is obvious, however, that such analysis cannot be applied for systems where all the hgands are not equal, i. e. MX Y 7- , and the ligands X and Y are very different from each other. 

For Te[S2P(OCH3)2l2 the ligand has been found to be monodentate 1 ). With zinc(II) and cadmium(II) complexes the ligand, O, O -di-isopropyl-dithiophosphate, has been shown to be both bidentate and bridging 178) xhe irregular pentagonal bipyramidal structure of... 

It is possible for an appropriately designed ligand to force a seven-coordinate pentagonal bipyramidal structure upon a V(III) complex. An example is the Schiff-base ligand shown in Fig. 4 [40]. In the [VLCl2]Cl complex the five nitrogen donor atoms are in a single plane and... 

Examples of monomeric Mo11 complexes are much less common. It has been demonstrated that the [Mo(CN)7]5 ion has a pentagonal bipyramidal structure. Other examples are nitrile (i.e. isocyanide) complexes [Mo(CNR)7]2+, which has a capped trigonal prismatic structure, and the diarsine complex [Mo(diars)2X2]. 

The H2NO ligand can be deprotonated to give HNO2 complexes. Thus, [M0(NO)(H2NO)(terpy)(H2O)]2+ and CN" yield a complex [Mo(NO)(HNO)(CN)-(terpy)]. The deprotonation is reversible and the proposed structure has been confirmed by X-ray analysis. In this case, too, the basic seven-coordinate pentagonal bipyramidal structure is observed, with NHO side-on, and NO axial, Z.M0NO = 178.0(7)°, Mo—N = 1.802(7) and N—O = 1.209(9) A.122... 

The reaction of Mo02(MeNHOi2 in the presence of the hydroxylamine ligand and CS2 proceeds according to Scheme 4. 4 The resultant formally seven-coordinate complex has a distorted pentagonal bipyramidal structure with the lone oxo group in the apical position. The five equatorial positions are occupied by two formally bidentate 0,N hydroxylamido ligands of... 

Noteworthy are the complexes obtained with the pentadentate ligands derived from the condensation of 2,6-diacetylpyridine with various substituted hydrazines (XI Table 98). These complexes have been found to be seven-coordinate with pentagonal bipyramidal structure where the pentadentate ligand invariably has its five donor atoms in the equatorial plane. The two residual coordination sites in the axial positions are occupied by two water molecules (325).2361-2364... 

A variety of organometallic compounds of Sn(IV) and Pb(IV) with 1,1-dithio ligands have been reported. The interesting MeSn(Et2Dtc)3 complex is cited (467) to have a pentagonal bipyramidal structure. The axial Sn-S distance opposite to the methyl group is 0.21 A shorter than the equatorial Sn-S distances. 

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