Experimental evidence in octahedral

Although the complexity increases rapidly there is no reason that Walsh diagrams cannot be constructed for XY3 pyramidal, XY4 tetrahedral, XYS octahedral, and other molecules. In fact, they have been prepared, but their applications will not be described here. Insofar as these diagrams are amenable to quantitative interpretation, the predictions are in accord with what we know from experimental evidence and valence bond methods. 

A less obvious conclusion, in terms of the experimental evidence available at the time, was that d6 pentacarbonyls such as Cr(CO)5 should prefer a square-based pyramidal geometry that is barely distorted away from the truncated octahedral structure. In other words, the CO ligands lying in the basal plane are at an angle of ca. 90° with respect to the axial CO. This geometry, rather than the one in which the basal CO ligands bend away from or toward the other CO, was predicted to be favored for reasons of orbital overlap. At the time of this publication, there was only limited experimental evidence from matrix isolation studies that this type of fragment had such a square-based pyramidal structure. 

The xenon atom has seven effective pairs of electrons surrounding it (one lone pair and six bonding pairs), one more pair than can be accommodated in an octahedral arrangement. Thus XeF6 should not have an octahedral structure but instead should be distorted from this geometry by the extra electron pair. There is experimental evidence that the structure of XeFg is not octahedral. 

On the other hand, there are good reasons for believing that this is not always the case and with, for example, antimony and bismuth, the heavier members of the group, there is evidence for the presence of an inert pair of s electrons. Because the angles between the substituents of a neutral antimony(III) or bismuth(III) compound are close to 90°, it is possible to consider that, rather than using sp hydrid orbitals, the substituents are attached via pure p orbitals, with the lone pair of electrons remaining localized in the appropriate s orbital. The concept of an inert pair of electrons has some validity and it allows rationalization of much of the chemistry of these elements in the + 3 oxidation state. It is, however, difficult to provide direct experimental evidence for the effect, but it is difficult otherwise to rationalize the almost ideal octahedral structure of [SbClg]. ... 

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