Formal negative charge

Now let s consider what happens when we have a carbon atom with a negative formal charge. The reason it has a negative formal charge is because it has one more electron than it is supposed to have. Therefore, it has live electrons. Two of these electrons form a lone pair, and the other three electrons are used to form bonds ... 

If oxygen has a negative formal charge, then it must have one bond and three lone pairs ... 

For each of the remaining nitrogen atoms, there is a negative formal charge. That means that each of those nitrogen atoms has one extra electron, 5 + 1=6 electrons. Each nitrogen atom has two bonds, which means that each nitrogen atom has... 

Negative formal charges normally reside on the atoms having higher electronegativity. 

Compare the total number of electrons that appear to be on each atom to the number of valence shell electrons that it normally has. If the number of electrons in the valence shell is greater than indicated in step 3, the atom appears to have lost one or more electrons and has a positive formal charge. If the number indicated in step 3 is larger than the number in the valence shell, the atom appears to have gained one or more electrons and has a negative formal charge. 

In this case, there is a simple experiment that will determine whether this is correct. Structure I places a negative formal charge on the terminal nitrogen atom, while structure II places a negative formal charge on the oxygen atom on the opposite end of the molecule. If the two structures contribute equally, these effects should cancel, which would result in a molecule that is not polar. In fact, the dipole moment of N20 is only 0.17 D, so stmctures I and II must make approximately equal contributions. 

The BH3 molecule is not stable as a separate entity. This molecule can be stabilized by combining it with another molecule that can donate a pair of electrons (indicated as ) to the boron atom to complete the octet (see Chapter 9). For example, the reaction between pyridine and B2H6 produces C5H5N BH3. Another stable adduct is carbonyl borane, OC BH3 in which a pair of electrons is donated from carbon monoxide, which stabilizes borane. In CO, the carbon atom has a negative formal charge, so it is the "electron-rich" end of the molecule. Because the stable compound is B2H6 rather than BH3, the bonding in that molecule should be explained. 

In each case, it is the carbon atom that has the negative formal charge that makes it the "electron-rich" end of the structure. Moreover, there are antibonding orbitals in both CO and CN that can accept electron density transferred from the nonbonding d orbitals on the metal (as described in Section 16.10). Because of the nature of these ligands, it is not uncommon for them to function also as bridging groups in which they are bonded to two metal atoms or ions simultaneously. 

Thus, starting from the electronic character of the heteroatom attached to the carbon chain, it is possible to assign positive and negative formal charges, in an alternate fashion, to all the carbon atoms of the molecular skeleton ... 

If the atom has more electrons in the molecule than when it is a free, neutral atom, then the atom has a negative formal charge, like a monatomic anion. If the assignment of electrons leaves the atom with fewer electrons than when it is free, then the atom has a positive formal charge, as if it were a monatomic cation. Mathematically, we write... 

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