Carbon Atoms with Formal Charges

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 ... 

Carbon has one more electron than boron, so the C—H moiety is isoelectronic with the B—H or BH2 moieties. Note that an isoelectronic relationship also exists between C and BH or B. In a formal sense it should be possible to replace a boron atom in a borune with a carbon atom (with an increase of one in positive charge) and retain an isoelectronic system. The best-studied system. C,B DH,2. is isoelectronic with [BI2H, ]3 and may be synthesized readily from decaborane and alkynes and dieihyl sulfide as solvent. 

The central carbon atom has four shared electrons in the covalent bonds to the two hydrogens, and two unshared electrons. The total number of electrons formally associated with the carbon in its valence shell is thus (4/2) + 2=4. This balances the 4 + charge that a carbon atom with no electrons in its valence shell would have, making it electrically neutral. 

A carbene is a molecule that contains a carbon atom with two bonds and two unpaired valence electrons. This leaves the carbon atom neutral in terms of formal charge (see Atoms and Molecules ), but still typically much more reactive than a typical carbon atom with four bonds. Carbenes are often found coordinated to metal centers in organometallic complexes. These carbene ligands are less reactive than a free carbene species, and actually you might be surprised to learn that organometallic carbene complexes aren t always prepared from the reactions of free carbenes with metal centers. As... 

An anion is electron rich and easily donates electrons to an electron-deficient species. A carbanion is a species in which a negative charge resides on carbon (carbon has a formal charge of -1), and it is electron donating. When a carbanion reacts with another carbon atom, it is classified as a nucleophile. 

A carbon atom with four bonds will always have no formal charge, but its oxidation state can... 

C bond with two oxygen atoms attached to each carbon atom) (b) BrO4 (c) the acetylide ion, C22. Assign formal charges to each atom. 

A formal charge is a charge associated with an atom that does not exhibit the expected number of valence electrons. When calculating the formal charge on an atom, we first need to know the number of valence electrons the atom is supposed to have. We can get this number by inspecting the periodic table, since each column of the periodic table indicates the number of expected valence electrons (valence electrons are the electrons in the valence shell, or the outermost shell of electrons— you probably remember this from high school chemistry). For example, carbon is in Column 4A, and therefore has four valence electrons. Now you know how to determine how many electrons the atom is supposed to have. 

Now we are in a position to compare how many valence electrons the atom is supposed to have (in this case, four) with how many valence electrons it actually has (in this case, four). Since these numbers are the same, the carbon atom has no formal charge. This will be the case for most of the atoms in the structures you will draw in this course. But in some cases, there will be a difference between the number of electrons the atom is supposed to have and the number of electrons the atom actually has. In those cases, there will be a formal charge. So let s see an example of an atom that has a formal charge. 

If carbon has a positive formal charge, then it has only three electrons (it is supposed to have four electrons, because carbon is in Column 4A of the periodic table). Since it has only three electrons, it can form only three bonds. That s it. So, a carbon with a positive formal charge will have only three bonds, and you should keep this in mind when counting hydrogen atoms ... 

From all of the cases above (oxygen, nitrogen, carbon), you can see why you have to know how many lone pairs there are on an atom in order to figure out the formal charge on that atom. Similarly, you have to know the formal charge to figure out how many lone pairs there are on an atom. Take the case below with the nitrogen atom shown ... 

When we treat all bonds as covalent, the carbon atom appears to have four electrons of its own. Carbon is supposed to have four valence electrons. When we compare how many electrons carbon actually has with the number of electrons it is supposed to have, we see that everything is just right in this case. It is supposed to have four valence electrons, and it is clearly using four valence electrons. Therefore, there is no formal charge. 

Each carbon atom has four bonds, and each oxygen atom has zero formal charge. The — CO2H group, with one CDO double bond and an acidic C—O—H linkage, is characteristic of carboxylic acids. 

Notice that the zinc atom is associated with only four valence electrons. Although this is less than an octet, the adjacent carbon atoms have no lone pairs available to form multiple bonds. In addition, the formal charge on the zinc atom is zero. Thus, Zn has only four electrons in the optimal Lewis structure of dimethyizinc. This Lewis stmcture shows two pairs of bonding electrons and no lone pairs on the inner atom, so Zn has a steric number of 2. Two pairs of electrons are kept farthest apart when they are arranged along a line. Thus, the C—Zn—C bond angle is 180°, and linear geometry exists around the zinc atom. 

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. 

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