Analogy with the octet rule

In the same way, one can construct a simplified interaction diagram for AH (or AR ) molecules in which A is an element from the second or third row of the periodic table (C, Si, N, P, O, S, etc ). There are thus four valence orbitals on the central atom one s AO, and three p AO. The a interactions with the orbitals on the atoms bound to A lead to the formation of n bonding MO, n antibonding MO and (4 — n) nonbonding MO. If the bonding and nonbonding MO are doubly occupied, the number of electrons Nt is equal to 

We have therefore derived the octet rule. As examples, we can quote  

It is Straightforward to verify that the number of bonding, nonbonding, and antibonding MO predicted by this simple model agree consistently with the detailed orbital structures of the NH3, OH2, and HE molecules ( 1.1.4-1.1.6). 

What are the two coordination modes of the allyl ligand, H2C-CH-CH2, for which a Lewis structure is given below In each case, categorize the ligand as LgXx. 

Wilkinsons catalyst [RhLsCl], where L=PPh3, is used for the hydrogenation of alkenes. The reaction proceeds as follows  

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By analogy with the octet rule, it has been proposed that a transition metal tends to be surrounded by the number of valence electrons equal to that of the following rare gas (electron configuration nd °(n + l)s (n + l)p ). One thereby obtains the 18-electron rule, for which we shall provide a first theoretical justification in this chapter ( 1.6.3). However, in light of the examples given above, one must note that there are many exceptions to this rule we shall analyse them in greater detail in the following chapters. 

An oversimplified rationale for the special significance of 18 electrons can be made by analogy with the octet rule in main group chemistry. If the octet represents a complete valence electron shell configuration then the number 18 represents a filled valence shell for a transition metal Although perhaps a useful way to relate electron... 

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