Inversion about pyramidal centers

Special cases of these involving transition states for rotation about single bonds, inversion of pyramidal nitrogen and phosphorus centers and ring inversion in cyclohexane, have been discussed in the previous chapter. The only difference is that these conformational processes are typically well described in terms of a simple motion, e.g., rotation about a single bond, whereas the motion involved in a chemical reaction is likely to be more complex. 

The molecules of the tetranuclear tin/cubane compound, tetrakis[(p.3-4-methoxy-2-methylphenylimido)tin(II)] (4), crystallizes as a 1 1.5 toluene solvate one of the toluene molecules is disordered about a center of inversion. In 4, an Sn4N4 cube with alternating tin and nitrogen atoms comprises the core. Each tin atom is therefore tri-coordinated by three nitrogen atoms in an approximately pyramidal fashion. In this example, there is no Sn... r interaction of note invoiving the solvent... 

As a freely ionic, monomeric species, the silyl anion may undergo pyramidal inversion about the silicon center (equation 1). For the parent system H3Si , Nimlos and Ellison have obtained quantitative information about the inversion barrier from the photoelectron spectrum in the gas phase2. The photoelectron spectrum could be simulated by a model of the vibrational frequency as a linear oscillator perturbed by a Gaussian barrier. The out-of-plane angle (the deviation of one H from the plane defined by Si and the other two Hs) was found to be 32 2° and the barrier to inversion 9000 2000 cm 1 (26 6 kcal mol-1). This is the only experimental measurement to date of the barrier to inversion about trivalent, negative silicon. The anion was produced by reaction of silane (SiH4) with ammonia, and the photoelectron spectrum of the m/z 31 peak was then recorded. 

The prototype structure of a simple, pyramidal carbanion has a lone pair of electrons in the (Tout orbital of Figure 1.8. An equivalent way to think about the structure is as an sp hybrid, placing the lone pair in an sp -like orbital. In most cases the barrier to inversion at a carbanion center is small, roughly 1-2 kcal/mol for simple systems. Thus, on most reaction time scales, carbanions behave as if they were effectively planar (Eq. 1.4). 

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