Pyramid structure isomers

Geometrical isomers are possible for complexes having a square-based pyramid structure. For example, the structures for a complex having the structure MLX2Y2 show that cis and trans arrangements are possible for the ligands in the base. 

Assume that the complex MLX2Y2 has a square-base pyramid structure with all ligands able to bond in all positions in the coordination sphere. How many isomers are possible ... 

An effect observed with a number of compounds which have apparent chiral centers on elements other than carbon. Eor example, secondary and tertiary amines have a pyramidal structure in which the unshared pair of electrons is at the top of the pyramid. If the three substituents hnked to the nitrogen are all different, one might suspect that the tertiary amine would give rise to optical activity and be resolvable. However, rapid oscillation of the unshared pair of electrons on one side of the nitrogen to the other (hence, pyramidal inversion) in effect causes interconversion of the two enantiomers and prevents resolution. If the nitrogen is at a bridgehead, this umbrella effect is inhibited and optical isomers can be isolated. 

The assignment of descriptors and configuration is sometimes arbitrary, at best, when based on model structures and pseudo-atom coordination numbers. A more explicit stereochemical notation is achieved by using this notation, which states within the stereodescriptor the model structure on which the notation is based. In this notation the pseudo-square pyramidal structure is [5PF-5-14-C(i )]. This structure can be expected to result in geometric isomers when one of the... 

A more common arrangement for EM4 species is the square pyramidal array with E occupying the apical vertex (51), which is related to the octahedral E2M4 (52) and EM5 (53) arrangements. Note that 51 is the structural isomer of 46 where the E atom adopts the apical rather than the basal position. These compounds are listed in Tables XVIII-XX, whereas Table XXI lists a number of more complicated molecules that have several... 

Fig. 10 Schematic drawings of the structures of cis and trans isomers of the complexes [Tc(N)C12(POP)] and [Tc(N)C12(PSP)]. The cis isomer exhibits a distorted square pyramidal structure with the two P atoms in a reciprocal cis position, whereas the trans isomer has a distorted trigonal bipyramidal structure with the two P atoms occupying the axial positions...

Recent molecular orbital (MO) calculations by Wade et al. (408) using the series [B Hg] , [BgH,]", and BgHg as models for the protonation of hexanuclear metal carbonyls have attempted to rationalize these findings. The charge distribution is symmetrical in an octahedral [BgHg] but asymmetrical in the capped square-based pyramidal isomer. It was found that upon protonation, significant charge redistribution occurs. This results in a substantial decrease in the symmetry of the octahedral cluster framework, which is disfavored in comparison with the capped square-based pyramidal structure much less affected by the protonation process. 

Further evidence for the stabilization of this structure in the Osg system comes from the fact that the octahedral isomer of Osg(CO)igH2, isolated by varying the conditions for protonation of [Osg(CO)ig] (433), slowly rearranges in solution to the more thermodynamically stable capped square-based pyramidal structure. The dihydride Rug(CO) gH2, however, does not show preference for this structure and retains the octahedral metal atom geometry observed for [Rug(CO),g] " and [Rug(CO)igH] (30,31,35) even on warming. 

The possible isomers are shown in Figure A.3. In fact, two isomers of this complex have been isolated and studied by X-ray diffraction methods. Both have a slightly distorted square pyramid structure, one being orange (A.3), the other violet (A.4). 

In Table 4, the energy differences AE between pyramidal and square isomers for MAI/1 complexes are shown. All of them show pyramidal structures lower in energy than the square planar and the AE lie in the range of 3.5 to 6kcal mol-1. 

Hardness values are computed from (2) for isomers of MA14" systems (Figure 4 and Table 4). In agreement with the energy differences, the pyramidal structures are always... 

As an example, for the oxidation of CO on Aug supported on magnesia surface, three different isomers are relevant. These structures are quasi planar, bicapped pyramidal (structures a and b in Fig. 1.59.), and bicapped octahedral. The quasiplanar structure is the lowest energy isomer. The bicapped pyramidal and the bicapped octahedral structures are higher in energy by 0.2 and 0.3eV, respectively. Upon adsorption of O2, the energetic order of the... 

The tricarboxylatostannates(II) show smaller chemical isomer shifts than the normal salts as indeed do the fluorides, chlorides, and phosphites [71]. The parameters show a small dependence on the cation. Once again a distorted pyramidal structure is thought probable. 

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