Quantum Mechanical Description of Donor-Acceptor Complexes

Mulliken has provided a quantum mechanical description of molecular complexes that are formed by reaction between an electron-donor (D) and an electron acceptor (A). The wave-function for the normal or ground-state of the complex may be expressed approximately according to Eqn. (1). 

Here we have retained only the first two terms of a more general expression for which has been described by MuUiken. The (D,A) and (D — A ) are called no-bond , and dative or charge-transfer structures, respectively. The designation of no-bond stracture for (D, A) refers to the absence of covalent bonding between the donor and the acceptor. 

In this Chapter, aspects of the electronic stmctures of complexes formed from n-type electron donors and sacrificial electron acceptors will be examined. An n-type donor donates an electron from essentially a lone-pair atomic orbital on a key atom, and a sacrificial acceptor accepts an electron into an antibonding molecnlar orbital. (MuUiken has also designated n-type donors as increvalent donors). Therefore, for this type of complex, D has a lone-pair of electrons and A has a vacant antibonding orbital in Eqn. (1). We shall assume here that the antibonding orbital of A extends over two adjacent atomic centres, and that the corresponding bonding molecular orbital of A is doubly occupied. 

Using our discussion above, and that of Chapter 14, we may deduce that the dative structure — A of Eqn. (1) (which arises from the transfer of an electron from a lone-pair atomic orbital on D into a vacant antibonding of A) corresponds 

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