Analytical calculations are performed for the bond orders in two- and four-electron three-centre model systems. It is shown that the presence of the two three-centre two-electron bonds in diborane leads to bond orders of about 0.5, not only between the "bridging" hydrogens and each boron atom, but also between the two borons; the corresponding attractive boron-boron exchange interaction stabilizes the diborane molecule against the dissociation into two BH3molecules. In accordance with the actual observations and Musher's theory, it is demonstrated that appreciable hypervalent bond order can be formed only with rather electronegative substituents withdrawing a considerable electron population from the central sulphur. It is concluded that the bond orders and bond lengths of such hypervalent bonds must be very sensitive to any changes in the group-electronegativity of the substituents; the increase in the electronegativity of one substituent leads to the strengthening of the hypervalent bond formed with the other, in full accord with the empirical observations. © 1989.
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