Identifying Incorrect Statement for Phosphomolybdate Anion

The question asks to identify the incorrect statement regarding the phosphomolybdate anion, $[\text{PMo}_{12}\text{O}_{40}]^{3-}$. Let's analyze each statement:

Statement 1: Keggin Structure Analysis

The phosphomolybdate anion, $[\text{PMo}_{12}\text{O}_{40}]^{3-}$, is the defining example of the Keggin structure type in polyoxometalates. Therefore, this statement is correct.

Statement 2: Phosphorus Oxidation State Calculation

To determine the oxidation state of Phosphorus (P), we use the known oxidation states of Molybdenum (Mo, typically +6 in polyoxometalates) and Oxygen (O, typically -2).

Let the oxidation state of P be $x$. The overall charge of the anion is -3.

The calculation is as follows:

Thus, Phosphorus is in the +5 oxidation state. This statement is correct.

Statement 3: Phosphomolybdate Anion Basicity Assessment

Polyoxometalates like $[\text{PMo}_{12}\text{O}_{40}]^{3-}$ contain highly oxidized metal centers (Mo(+6)) and electronegative oxygen atoms. These features impart acidic properties (both Lewis and Brønsted acidity) to the anion in aqueous solution. They are not basic.

Therefore, the statement "It is extremely basic" is incorrect.

Statement 4: Precipitation with Tetraalkylammonium Ions

The large, bulky tetraalkylammonium cations ($[\text{R}_4\text{N}]^+$, where R is an alkyl or aryl group) are often used to precipitate polyoxometalate anions from aqueous solutions. Their size and hydrophobic nature facilitate the formation of insoluble salts with these highly charged anions.

This statement is correct.

Conclusion

Based on the analysis, the incorrect statement about the phosphomolybdate anion, $[\text{PMo}_{12}\text{O}_{40}]^{3-}$, is that it is extremely basic.