Octet rule violations

For a general closed-shell AX , species, the Lewis-type assumption of a shared A X electron-pair bond for each coordinated monovalent atom X nominally requires m orbitals on A to accommodate the 2m bonding electrons, plus additional orbitals for any nonbonded pairs. Thus, for m bonds and t lone pairs, apparent octet-rule violations occur whenever... 

Protonated methane (CH ) does not violate the octet rule of carbon. A bonding electron pair (responsible for covalent bonding between C and H atoms) is forced into sharing with the proton, resulting in 2 electron-3 center bonding (2e-3c) (see Chapter 10). Higher alkanes are protonated similarly. 

First, look at the reaction and identify the bonding changes that have occurred. In this case, a C—Br bond has broken and a C-C bond has formed. The formation of the C-C bond involves donation of an electron pair from the nucleophilic carbon atom of the reactant on the left to the electrophilic carbon atom ol CH Br, so we draw a curved arrow originating from the lone pair on the negatively charged C atom and pointing to the C atom of CH3Br. At the same time the C—C bond forms, the C-Br bond must break so that the octet rule is not violated. We therefore draw a second curved arrow from the C-Br bond to Br. The bromine is now a stable Br- ion. 

There are a few species in which the central atom violates the octet rule in the sense that it is surrounded by two or three electron pairs rather than four. Examples include the fluorides of beryllium and boron, BeF2 and BF3. Although one could write multiple bonded structures for these molecules in accordance with the octet rule (liable 7.2), experimental evidence suggests the structures... 

The largest class of molecules to violate the octet rule consists of species in which the central atom is surrounded by more than four pairs of valence electrons. Typical molecules of this type are phosphorus pentachloride, PC15, and sulfur hexafluoride, SF6. The Lewis structures of these molecules are... 

From now on, we will refer to the second commandment as the octet rule. But be careful—for purposes of drawing resonance structures, it is only a violation if we exceed an octet for a second-row element. However, there is no problem at all with a second-row element having fewer than an octet of electrons. For example ... 

This drawing is perfectly acceptable, even thongh the central carbon atom has only six electrons surrounding it. For our purposes, we will only consider the octet rule to be violated if we exceed an octet. 

Onr two commandments (never break a single bond, and never violate the octet rule ) reflect the two parts of a curved arrow (the head and the tail). A bad tail violates the first commandment, and a bad head violates the second commandment. 

Now we need to ask if the second commandment has been violated did we violate the octet rule To determine this, we look at the head of the arrow. Are we forming a fifth bond Remember that C+ only has three bonds, not four. When we push the arrow shown above, the carbon atom will now get four bonds, and the second commandment has not been violated. 

This does not violate either of the two coimnandments. We did not break any single bonds and we did not violate the octet rule. So this is a valid structure. Notice that we cannot move the lone pair in another direction, because then we would be violating the octet rule ... 

This violates the octet rule—the carbon atom would end up with live bonds. So we cannot push the arrows that way. There is no way to turn the lone pair into a pi bond in this example. 

The arrow on the top structure violates the octet rule (giving carbon five bonds), and the arrow on the bottom structure does not violate the octet rule. The arrow on the bottom structure will therefore provide a valid resonance structure ... 

Now that we have learned all three steps, we need to consider that these steps can be combined. Sometimes we cannot do a step without violating the octet rule, but by doing two steps at the same time, we can avoid violating the octet rule. For example, if we try to turn a lone pair into a bond in the following structure, we see that this would violate the octet rule ... 

In other words, you should not always jump to the conclusion that pushing an arrow will violate the octet rule. You should first look to see if you can push another arrow that will eliminate the problem. 

Now let s do step 1 can we convert any lone pairs into pi bonds If we try to bring down the lone pairs, we will violate the octet rule by forming a carbon atom with five bonds ... 

Once you learn to recognize this pattern (a lone pair next to a pi bond), you will be able to save time in calculating formal charges and determining if the octet rule is being violated. You will be able to push the arrows and draw the new resonance structure without thinking about it. 

This might seem better at first, because we get rid of the charges, but our two commandments show us why it cannot be drawn like this the nitrogen atom would have five bonds, which would violate the octet rule. 

Note that violations of the octet rule by the central atom occur with atoms such as sulfur and phosphorus. These are atoms that have d orbitals as part of their valence shells, so they are not limited to... 

B) To form OF there would have to be six bonds (twelve electrons around the oxygen atom). This would violate the octet rule. O does not have an empty d sublevel into which it can form expanded octets. S has an empty 3d sublevel that it uses to form six bonds in SF. ... 

B is the only central atom among the choices that violates the octet rule by having only 6 valence electrons in BCI3. (D) Any gas A that is lost results in equal amounts of gas B and gas C. Therefore,... 

Molecules comprising first-row elements (C-F) only nearly always satisfy the octet rule, that is, have eight valence electrons around each first-row atom. Violations occur for molecules incorporating elements on left-hand side of the Periodic Table, e.g., B, where there may not be sufficient electrons to satisfy the overall demand. Very seldom are molecules encountered with more than eight valence electrons. 

The Lewis dot structure for BF3 shows the central boron atom being surrounded by only six electrons, which violates the octet rule. This illustrates that the octet rule, while providing general guidelines, has exceptions. 

Some atoms take on more than a full octet s worth of electrons. These atoms are said to be hypervalent or hypercoordinated. The phosphorus of phosphorus pentachloride, PCI5, is an example. These kinds of situations require an atom from Period (row) 3 or higher within the periodic table. The exact reasons for this restriction are still debated. Certainly, the larger atomic size of these atoms allows room to accommodate the bulk of all the binding partners that distribute around the central atom s valence shell. In some cases, even noble gases like xenon (Xe) form compounds. Because noble gases already have a filled valence shell, they automatically violate the octet rule. 

BC13, C1F3, nd PCI ) the octet rule has been violate it is not possible to rectify this because the ligands (a... 

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