Valence bond description coordinate bonds

Figure 5-20. Reaction coordinate diagram generated from a valence bond description of initial and final state configurations.

A SIMPLE VALENCE BOND DESCRIPTION OF COORDINATE BONDS... 

A Simple Valence Bond Description of Coordinate Bonds 593... 

We have discussed alkene coordination from a molecular orbital point of view. However, many of the conclusions also follow from a valence bond description comprising the two canonical forms shown in Figure 1.19. The nature of the metal and alkene substituents will dictate the... 

Fiorillo, A.A. and Galbraith, J.M. (2004) A valence bond description of coordinate covalent bonding. J. Phys. Chem. A, 108, 5126-5130. 

It would appear that for a description of the four electron system it would be necessary to consider the superposition of the three given structures with different localizations of the valency bonds. However, it is still possible to make a further simplification. In the problem of three electrons, three structures also were possible with the bond between a and b and with a free atom c (spin function aj8a—jSaa) with a bond between atoms b and c and a free atom a (spin function aajS—ajSa) with the bond between atoms a and c and a free atom b (spin function aajS—jSaa). But we pointed out that these three functions were not independent, the third being a linear combination of the other two. Let us write down the appropriate functions for the four electron problems describing the states /, II and IIL The space coordinate part of any wave function of four electrons will have the form... 

The N=N=0 (I) molecule has the first example in this series of a central nitrogen atom which, in order to satisfy all the usual valencies of the atoms bonded to it, would have to be described as a five-coordinate nitrogen atom. The proper description of such molecular systems, which are commonly called 1,3 dipoles because of their chemistry of 1,3 addition to double bonds, has been discussed extensivelyConsensus opinion is that NNO, for example, has both a fully formed double bond and a fully formed triple bond. Specifically, the central nitrogen atom participates in five fairly normal two-electron covalent bonds. The central nitrogen atom in N2O can therefore legitimately be called hypervalent. 

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