Increased-Valence Structures with three 2-Centre Bond Orbitals

For the hydrogen bonding interaction between two water molecules we may write 

In the theory of Sections 20-1 to 20-3, the electron donor (X) and the electron acceptor (R—Y) were designated as increvalent and sacrificial donors and acceptors respectively. Here, in structures (12), (13) and (14), donors and acceptors are both increvalent species. 

20-5 Increased-Valence Structures with three 2-Centre Bond Orbitals 

An interesting result is obtained if 2-centre bond-orbitals are used to accommodate all of the bonding electrons in the increased-valence structure (13) and the standard Lewis stmcture (15). We shall designate these bond orbitals as =x + lr and tgjy =v + ky, with the bond parameters I and k both 0. The 

The wave-function for the dative structnre X —RY, for which one nonbonding electron of stmctnre (15) has been transferred into the antibonding RY orbital )/ = r - y, is given by Eqn. (6). 

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Although 3-electron 3-centre bonding nnits with three overlapping atomic orbitals are not electron-rich, for them increased-valence structures of the general types X R Y and X R Y (with atomic formal charges omitted) can be constructed. With one atomic orbital (x, r and y) per atomic centre, their electrons occupy the orbitals of Eqs. (2) and (3), respectively. 

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