Question

The compound(s) that does(do) not exist is(are)

• A. BiF$_5$
• B. PF$_5$
• C. AsF$_5$
• D. SbF$_5$
• E. All the compounds exist

Hint:

BiF$_5$ is the only stable pentahalide of Bismuth. Heavier halogens cannot stabilize the +5 oxidation state of Bismuth due to its high oxidizing power and the large size of the halogens.

Answer 1

Answer: (E)

Concept: The stability of Group 15 pentahalides depends on the ability of the central atom to undergo $sp^3d$ hybridization and expand its octet. As we move down the group, the "inert pair effect" makes the +5 oxidation state less stable, but Fluorine's high electronegativity can often stabilize it.

Step 1: {Check PF$_5$, AsF$_5$, and SbF$_5$.}
Phosphorus, Arsenic, and Antimony all have available d-orbitals and can easily form stable pentahalides with highly electronegative atoms like Fluorine. These compounds are well-known and stable.

Step 2: {Evaluate the existence of BiF$_5$.}
Bismuth (Bi) is at the bottom of Group 15. Due to the inert pair effect, the +3 oxidation state is generally more stable than +5. However, because Fluorine is the most electronegative and smallest halogen, it is capable of oxidizing Bismuth to its +5 state.

Step 3: {Conclude the existence.}
Unlike BiCl$_5$ or BiI$_5$ (which do not exist), BiF$_5$ is a known, stable crystalline solid. Therefore, all the listed pentafluorides in the options exist under standard conditions.