Hybridisation theory has failed in a few aspects, notably in explaining the energy considerations for the involvement of d-orbitals in chemical bonding. This can be well-explained by means of an example. Consider, for instance, how the theory in question accounts for the bonding in phosphorus pentachloride (PCl5). The d-orbitals are large, comparatively distant from the nucleus and high in energy. Radial distances of orbitals from the nucleus seem to reveal that d-orbitals are far too high in energy to 'mix' with s- and p-orbitals. 3s - 0.47 , 3p - 0.55, 3d - 2.4 (in angstroms). Thus, at first glance, sp3d hybridisation seems improbable. However, a closer examination of the factors that affect orbital size (and energy) reveals more. Formal charge on the central atom is one such factor, and it is obvious that the P atom in PCl5 carries quite a large partial positive charge. Thus the 3d orbital contracts in size to such an extent that hybridisation with s and p orbitals may occur. Further, note the cases in which d-orbital participation was proposed in hybridisation: SF6(sulfur hexafluoride), IF7, XeF6; in all these molecules, the central atom is surrounded by the highly electronegative fluorine atom, thus making hybridisation probable among s, p and d orbitals. A further study reveals that orbital size also depends on the number of electrons occupying it. The least stable molecule among the following isSH6 does not exist becauseThe most probable element(X) with which silicon can form an ion of the stoichiometry SiX62- and the hybridization of silicon in it is