Assertion: Zero and negative oxidation state of d-block metal ion are not possible in their complex compound.
Reason: Low oxidation state of the metal ions are found when a complex compound has ligands capable of `pi-`aceptor character in addition to the `sigma-` bonding.

Options

A. If both assertion and reason are true and the reason is the correct explanation of assertion
B. If both assertion and reason are true and the reason is not the correct explanation of assertion
C. If assertion is false but reason is true
D. If assertion is true but the reason is false

Explanation

To analyze the given assertion and reason, we will break down the concepts involved and assess their validity step by step.

### Step 1: Understanding the Assertion
**Assertion:** Zero and negative oxidation states of d-block metal ions are not possible in their complex compounds.

- **Explanation:** In general, d-block metals can exhibit a variety of oxidation states due to the involvement of d-orbitals in bonding. However, the assertion claims that zero and negative oxidation states are not possible in complex compounds. This statement is incorrect because certain ligands can stabilize low oxidation states, including zero and negative.

**Conclusion for Step 1:** The assertion is **false**.

### Step 2: Understanding the Reason
**Reason:** Low oxidation states of the metal ions are found when a complex compound has ligands capable of π-acceptor character in addition to the σ-bonding.

- **Explanation:** Ligands that possess π-acceptor character, such as carbon monoxide (CO) and nitrogen oxides (NO), can stabilize low oxidation states of d-block metals. These ligands can engage in back-bonding, where the metal can donate electron density into the empty π* orbitals of the ligand, thus stabilizing the metal in a lower oxidation state.

**Conclusion for Step 2:** The reason is **true**.

### Step 3: Evaluating the Relationship
Now that we have established the truth values of both the assertion and the reason, we need to evaluate their relationship:

- The assertion is **false**.
- The reason is **true**.

Based on the options provided:
- A: Both assertion and reason are true, and the reason is the correct explanation of the assertion.
- B: Both assertion and reason are true, and the reason is not the correct explanation of the assertion.
- C: Assertion is false, but reason is true.
- D: Assertion is true, but reason is false.

Since the assertion is false and the reason is true, the correct choice is **C**.

### Final Answer
**C: Assertion is false, but reason is true.**

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