Tetragonal-pyramidal molecules

Let us examine the properties of molecules where a central atom M is surrounded by five ligands A, B, C, D, E. We assume that the ligands are at the vertices of a trigonal bipyramid. This assumption is adequate for most pentacoordinate complexes but we ougth to mention that the description of stereoisomerization we propose could be applied if another polytopal form—the tetragonal pyramid, for example—was the stable one. The same type of description has already been undertaken for hexacoordinate octaedral complexes . ... 

The coordination environment of the two copper atoms is tetragonal pyramidal and the carbonate coordinates along the basal plane. In the molecule both the N4 coordinated Cu2+ ions are bridged by carbonate in a bidentate fashion (see E). The Cu Cu distances in the molecules D, and D2 are 4.303 and 4.548 A, respectively, and the phen ligands show strong stacking interaction in the crystal. 

The X-ray crystal structures of a number of compounds of general formula [PdBr2L3] have been determined, for example when L = 2-phenylisophosphinoline (2), the structures of both racemic and optically resolved forms were determined," and when L = (3), the structure of [PdBr2L3]-PhCl was obtained.26 The geometry of all these molecules is similar, being distorted tetragonal pyramidal with an apical bromine. 

The coordination of bismuth atoms varies considerably in the structures described in the literature, but as for all other derivatives of Bi(III) these are the geometries with a localized electron pair [Bi ] that are dominating. The minimal CN 3 and 4 ( /-tetrahedron and -trigonal bipyramid) are present in the molecule ofBi(OBu )3 (in the gas phase) and in the polymeric structure of [KBi(ji-OBu,)4] . There are also present the structures with 5-coordinated Biatom (tetragonal pyramid = y-octahedron). Its representatives are the structure... 

Figure 7.4. A projection along c0 of the atoms in the tetragonal cell of B5H9. Boron atoms are the smaller circles. The centers of the four B atoms in the square base of the pyramidal molecules are at 0 (and 100) and 50 giving a body-centered cell.

When, as in spiroarsoranes of type 138, an additional benzo-anneland across the 3,4-positions of one 2,2 -biphenylylene ligand renders the potential tetragonal-pyramidal transition state 137 highly improbable because of prohibitive steric overcrowding, inner flexibility of the molecule can express itself now only by... 

When a molecule is relatively small and/or belongs to a point group of relatively high symmetry, it is possible to elucidate the molecular structure by using the symmetry selection rules discussed in Section 1.14. Molecules of XY2 (linear or bent C2v), XY3 (planar D3h or pyramidal C3v), XY4 (square-planar D4h or tetrahedral Td) and XY5 (trigonal-bipyramidal D3h or tetragonal-pyramidal C4v) types may take one of the structures indicated in parentheses. Since the number of IR/Raman-active vibrations is different for each structure, the most probable structure can be chosen by comparing the number of observed IR/Raman bands with that predicted for each structure by symmetry selection rules. 

What is chirality The term chirality denotes the property of a molecule to be nonidentical to its mirror image. All coordination polyhedrons with identical ligands are achiral. When the ligands are different, the chiral compounds may be formed. Thus, a tetrahedron with four different ligands is chiral. A trigonal bipyramid and tetragonal pyramid may be (but not necessarily will be) chiral if three (or more) ligands are of different types, e.g. 

SbCb is a weak electron pair donor but an excellent acceptor and many complexes of SbCb with donor molecules L are known." " There are complexes of the types LSbCls or L2SbCl3 with bisphenoidal or tetragonal pyramidal stmctures respectively. Addition of CL to SbCb gives bisphenoidal... 

In the transition group complexes containing a partly filled d shell, it is possible that a reminiscent Gillespie-effect contributes to the intensity of Laporte-forbidden transitions. Thus, the absorption band of Co(NH3)6 has roughly the same oscillator strength as -that of Co(NH3)5C1+2, and there is some evidence (54) that Cu is above the plane of the four ammonia molecules, not only in the tetragonal-pyramidic Cu(NH3)5+2, but also in an instantaneous picture in Cu(NH3)4 2 which would constitute essentially a Gillespie effect. 

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