Octet rule electron-deficient compounds

There are a number of general exceptions to the octet rule electron deficient compounds in which the central atom has an incomplete octet, free radicals (with an unpaired electron), molecules whose central atom is surrounded by more than eight electrons (an expanded octet) and compounds of hydrogen and the transition metals (d-block) and lanthanoids (f-block). 

In BeCl2, the chlorine atoms achieve the argon configuration, [Ar], and the beryllium atom has a share of only four electrons. Compounds such as BeCl2, in which the central atom shares fewer than 8 e, are sometimes referred to as electron deficient compounds. This deficiency refers only to satisfying the octet rule for the central atom. The term does not imply that there are fewer electrons than there are protons in the nuclei, as in the case of a cation, because the molecule is neutral. 

The LE model is a simple but very successful model, and the rules we have used for Lewis structures apply to most molecules. To implement this model we have relied heavily on the octet rule. So far we have treated molecules for which this rule is easily applied. However, inevitably, cases arise where the importance of an octet of electrons is called into question. Boron, for example, tends to form compounds in which the boron atom has fewer than eight electrons around it—it does not have a complete octet. Boron trifluoride (BF3), a gas at normal temperatures and pressures, reacts very energetically with molecules such as water and ammonia that have available lone pairs. The violent reactivity of BF3 with electron-rich molecules occurs because the boron atom is electron-deficient. Boron trifluoride has 24 valence electrons. The Lewis structure that seems most consistent with the properties of BF3 is... 

Since the number of electrons required to achieve Nirvana is so large for transition metal species, the stability differences of other electron counts are not too forbidding. Consequently, the 18e rule is softer than the octet rule, and we may expect to find relatively persistent radical complexes with 17 electrons, and complexes with 16e or even less. These cases are in fact extremely interesting because the electron-deficient complexes can serve as catalysts that activate other molecules. Despite all these qualifications, the Law of Nirvana for transition metal compounds is a very useful guide for constructing transition metal complexes and for considering their reactivity (propensity to react) and properties. Let us see how the rule is applied along with the click bond method. 

This electron-deficient unit does not obey the octet rule, as it does not have enough electrons to form four two-centre two-electron bonds around each carbon atom. The interaction between an sp hybrid orbital of a methyl group with three 2s orbitals of the Li atoms form a symmetric four-centre two-electron bonding orbital, which accounts for the bonding in this compound. 

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