Incomplete octets

Boron forms perhaps the most extraordinary structures of all the elements. It has a high ionization energy and is a metalloid that forms covalent bonds, like its diagonal neighbor silicon. However, because it has only three electrons in its valence shell and has a small atomic radius, it tends to form compounds that have incomplete octets (Section 2.11) or are electron deficient (Section 3.8). These unusual bonding characteristics lead to the remarkable properties that have made boron an essential element of modern technology and, in particular, nan otechn ol ogy. 

Boric acid, B(OH) , is a white solid that melts at 171°C. It is toxic to bacteria and many insects as well as humans and has long been used as a mild antiseptic and pesticide. Because the boron atom in B(OH)3 has an incomplete octet, it can act as a Lewis acid and form a bond by accepting a lone pair of electrons from an H20 molecule acting as a Lewis base ... 

The B atom has an incomplete octet in all its trihalides. The compounds consist of trigonal planar molecules with an empty 2p-orbital perpendicular to the molecular plane. The empty orbital allows the molecules to act as Lewis acids, which accounts for the catalytic action of BF3 and BC13. 

Boron and aluminum halides have incomplete octets and act as Lewis acids. 

The nitrogen atom in the N02 molecule has an incomplete octet, having a single unpaired electron. The unpaired electrons of the nitrogen atoms combine to form a single bond. 

Hydrogen bonding is so common that coordinate bonds between other elements are sometimes overlooked. Antimony(Ill) halides form very few complexes with other halides, whereas aluminum halides readily form complexes. The octet of electrons is complete in all atoms of the antimony halides, but is incomplete in die aluminum atom of aluminum halides ... 

Compounds of boron and aluminum may have unusual Lewis structures in which boron and aluminum have incomplete octets or halogen atoms act as bridges. 

Incomplete Octets. Even though the formation of the maximum number of covalent bonds is a powerful driving force in the formation of molecules, there are instances of molecules where not all of the available orbitals are used. This then will lead to structures where less than four electron pairs surround an atom. 

Sometimes molecular species with incomplete octets are en-... 

The reaction sequence shown above illustrates three important aspects of chemistry that will be shown to be very important in the discussion of atmospheric chemistry in Section 2.8. The first of these is that a reaction may be initiated by a photochemical process in which a photon of light (electromagnetic radiation) energy produces a reactive species, in this case the Cl- atom. The second point illustrated is the high chemical reactivity of free radical species with unpaired electrons and incomplete octets of valence electrons. The third point illustrated is that of chain reactions, which can multiply manyfold the effects of a single reaction-initiating event, such as the photochemical dissociation of Cl2. 

Covalent chemistry is the characteristic property of non-metals. Hydrogen can form only one covalent bond and in boron compounds we find an incomplete octet. 

Some molecules containing boron (e.g. BF3) have an incomplete octet (only six electrons with B) and these effect the chemical properties of such compounds. 

In the cases above, each pair of electrons corresponds to a univalent bond. If a greater number of electrons is lacking, the incomplete spheres can attain the octet configuration by sharing two or three pairs of electrons. In a molecule of carbon dioxide the carbon is bonded with two oxygen atoms by two double valence bonds ... 

Empirically measured parameters are additional solvent properties, which have been developed through the efforts of physical chemists and physical organic chemists in somewhat different, but to some extent related, directions. They have been based largely on the Lewis acid base concept, which was defined by G. N. Lewis. The concept originally involved the theory of chemical bonding which stated that a chemical bond must involve a shared electron pair. Thus, an atom in a molecule or ion which had an incomplete octet in the early theory, or a vacant orbital in quantum mechanical terms, would act as an electron pair acceptor (an acid) from an atom in a molecule or ion which had a complete octet or a lone pair of electrons (a base). Further developments have included the concepts of partial electron transfer and a continuum of bonding from the purely electrostatic bonds of ion-ion interactions to the purely covalent bonds of atoms and molecules. The development of the concept has been extensively described (see Ref. 11 for details). 

There are a few notable exceptions to the octet rule that you should be aware of molecules with odd numbers of electrons, incomplete octets, and expanded octets. 

In the previous example, note that the success of the reaction depended upon the acid (BF3) having an incomplete octet. Many substances, including metal ions that have incomplete octets, can act as Lewis acids in the presence of Lewis bases. 

Which of the following has an incomplete octet in its Lewis structure ... 

The correct answer is (D). BF3, with its incomplete octet, is capable of accepting a pair of electrons from F to form a covalent bond. Electron pair acceptors are Lewis... 

One thing to look out for whenever you see ammonia is its behavior as a Lewis base and its ability to form complexes. In this problem, you are also given boron trichloride, which is frequently cited as being an example of a molecule where the central atom has an incomplete octet. Although boron trichloride is stable, it readily reacts with ammonia. 

Atoms with an incomplete octet can likewise form bonds with atoms with lone electron pairs. Thus similarly to the onium compounds, the amine oxides, the sulphoxides and sul-phones are produced by addition of an oxygen atom to the nitrogen or sulphur atom of amines or sulphides. 

There are a few relatively stable species in which, formally, the octet of nitrogen is incomplete. The classic examples are NO and N02 together with nitroxides R2NO and the ion (03S)2N02 all have one unpaired electron. Nitroxide radicals are used... 

In later sections we shall use certain other criteria to help us estimate stabilities of possible contributing structures (b) number of bonds (Sec. 8.17) (c) dispersal oj charge (Sec. 11.19) (d) complete vs. incomplete octet (Sec. 11.20) (e) separation of charge (Sec. 18.12). 

The Unusual Structures of Some Group 13/III Compounds Incomplete octet... 

The single-bonded structure with an incomplete octet makes the major contribution. 

Other compounds with incomplete octets are BCI3 and AICI3 vapor at high temperature... 

Example Donation of a lone pair by one atom to create a bond that completes the octet of another atom with an incomplete octet. In the Lewis structure for the reaction of BF3 with NH3, both electrons in the B-N bond are derived from the N lone pair. 

Boron, like most nonmetals, has acidic oxides. Hydration of B2O3 yields boric acid, H3BO3 or B(0H)3, in which B has an incomplete octet and can therefore act as a Lewis acid ... 

All boron halides are trigonal-planar, covalently bonded, and electron-deficient, with an incomplete octet on B, as a consequence of which, they are strong Lewis acids. 

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