Nuclear energy deshielded

Tin(IV) compounds are known with tin coordination numbers of 3,4, 5, 6, and in some cases >6. The search for free triorganotin cations (stannylium ions) continues, and a few examples (e.g. 25 and have been reported where three-coordinate tin atoms are likely to be present both in the solid state and in solution (Scheme 2.1.6). The extreme " Sn nuclear deshielding in the tris(silyl)stannylium ion 26 is caused by Bo-induced currents involving a -tt magnetic dipole-allowed transitions, for which the energy difference is much smaller when compared with the triarylstannylium ion 25. Examples of another new class of organotin compounds are also shown in Scheme 2.1.6. The positively charged tin centers in these zwitterionic species 27 and 28 are stabilized by coordination with C C bonds. ... 

The nature of the repulsive force that arises when two closed-shell systems approach each other can be described in many ways, one of which is as follows. At larger distances the electron clouds of two atoms attract each other by dispersion forces, while at short distances the Pauli exclusion principle drives electrons with the same quantum numbers away from the space they are trying to share, so that a local deshielding of the nuclei occurs, and repulsion arises. Note that the Pauli principle does not imply forces , but only the purely quantum mechanical effect of mutual electron avoidance. At equilibrium, which results from a balance between dispersion attraction and nuclear repulsion forces, there is usually a net gain in energy. The parametric potential must therefore consist of a repulsive (the exponential) and an attractive (mostly, m = 6) term. 

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