Certain compounds do not conform to the octet rule due to the existence of odd-electron species, expanded octets, or incomplete octets.

The octet rule is a fundamental principle in chemistry that suggests that atoms of main-group elements tend to bond in such a manner that each atom attains eight electrons in its valence shell. This configuration is akin to that of noble gases, which are known for their stability. Nonetheless, there are notable exceptions to this rule, primarily arising from odd-electron species, expanded octets, and incomplete octets.

Odd-electron species, often referred to as free radicals, are molecules or ions that contain an unpaired electron. These species do not adhere to the octet rule because their total number of electrons is odd, making it impossible to distribute them evenly among the constituent atoms while ensuring that each atom achieves a complete octet. Examples of odd-electron species include nitric oxide ($\text{NO}$) and the methyl radical ($\text{CH}_3$).

Expanded octets are observed in molecules or ions that possess more than eight valence electrons surrounding the central atom. This phenomenon occurs in elements from the third period and beyond, as these elements have available d orbitals in addition to their s and p orbitals. This enables them to accommodate more than eight electrons. Notable examples of expanded octets include phosphorus pentachloride ($\text{PCl}_5$) and sulfur hexafluoride ($\text{SF}_6$).

Incomplete octets occur in molecules or ions that have fewer than eight electrons surrounding an atom. This situation is frequently seen in elements that possess fewer than four valence electrons, such as hydrogen, helium, lithium, beryllium, and boron. These elements can form stable compounds despite having incomplete octets. For instance, in boron trifluoride ($\text{BF}_3$), the boron atom is surrounded by only six electrons in its valence shell.

In summary, while the octet rule serves as a valuable guideline for understanding the structure and behavior of many chemical compounds, it is not universally applicable. The presence of odd-electron species, expanded octets, or incomplete octets can lead to significant deviations from the octet rule in various compounds.