Regarding the oxidation states of elements of transition element the incorrect statement is

Options

A. `Mo^(+6)` is more stable than `Cr^(+6)`
B. `W^(+6)` is more stable than `Cr^(+6)`
C. Oxoanion of `Cr^(+6)` in acidic medium is better oxidizing agent than oxides of Mo and W in + 6 oxidation state .
D. Higher oxidation states are shown by metals when they are attached to `pi` - acceptor ligands .

Answer

To solve the question regarding the incorrect statement about the oxidation states of transition elements, we will analyze each statement provided in the context of transition metals, particularly focusing on chromium (Cr), molybdenum (Mo), and tungsten (W).

### Step-by-Step Solution:

1. **Understanding the Stability of Oxidation States:**
- Transition metals can exhibit multiple oxidation states due to the involvement of their d and s orbitals. The stability of these oxidation states can vary depending on the element and its position in the periodic table.

2. **Analyzing the First Statement:**
- The first statement claims that Mo +6 is more stable than Cr +6.
- As we move down a group in the periodic table, the stability of higher oxidation states generally increases. Therefore, Mo +6 is indeed more stable than Cr +6. This statement is **true**.

3. **Analyzing the Second Statement:**
- The second statement claims that W +6 is more stable than Cr +6.
- Similar to the first statement, W is below Mo in the periodic table, and thus W +6 is also more stable than Cr +6. This statement is **true**.

4. **Analyzing the Third Statement:**
- The third statement asserts that the oxoanion of Cr +6 in acidic medium is a better oxidizing agent than the oxoanions of Mo +6 and W +6.
- Cr +6 is less stable compared to Mo +6 and W +6, which means it is more likely to be reduced to a lower oxidation state. This makes Cr +6 a better oxidizing agent than the other two. This statement is **true**.

5. **Analyzing the Fourth Statement:**
- The fourth statement claims that higher oxidation states are shown by metals when they are attached to π-acceptor ligands.
- In reality, π-acceptor ligands stabilize lower oxidation states of metals due to synergic bonding. Therefore, this statement is **incorrect**.

### Conclusion:
The incorrect statement regarding the oxidation states of transition elements is the one that claims higher oxidation states are shown by metals when they are attached to π-acceptor ligands.