Ligands are broadly classified into two classes classical and non-classical ligands, depending on their donor annd acceptor ability. Classical ligands form classical complexes while non-classical ligands form non-classical complex. Bonding mechanism in non-classical is called synergic bonding.
Q. In compound `[M(CO)_(n)]^(z)`, the correct match for highest 'M-C' bond length for given M, n and z respectively:

Options

A. M-Cr, n-6, z-0
B. M-V, n-6, z- `-1`
C. M-Ti, n-6, z- `-2`
D. M-Mn, n-6, z- `+1`

Explanation

To solve the problem regarding the highest 'M-C' bond length in the compound `[M(CO)_(n)]^(z)`, we will analyze the oxidation states of the metals and their corresponding electron densities. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Metal and Ligands
The metal in question is chromium (Cr) with six carbonyl (CO) ligands attached to it. The charge on the complex is zero.

**Hint:** Remember that carbonyl (CO) is a neutral ligand, which means it does not contribute to the overall charge of the complex.

### Step 2: Determine the Oxidation States
For each metal complex, we need to calculate the oxidation state of the metal:
- For chromium in `[Cr(CO)6]`, the oxidation state is 0 (since CO is neutral).
- For vanadium in `[V(CO)6]^-1`, the oxidation state is -1.
- For titanium in `[Ti(CO)6]^-2`, the oxidation state is -2.
- For manganese in `[Mn(CO)6]^-1`, the oxidation state is +1.

**Hint:** Use the formula: Oxidation state of metal (X) + (number of ligands × charge of ligands) = overall charge of the complex.

### Step 3: Analyze Electron Density and Bonding
The concept of synergic bonding is crucial here. In carbonyl complexes, electron density on the metal affects the strength of the M-C bond:
- Higher electron density on the metal leads to stronger M-C bonds and shorter bond lengths.
- Conversely, lower electron density results in weaker M-C bonds and longer bond lengths.

**Hint:** Remember that the oxidation state of the metal is inversely related to its electron density; higher oxidation states typically mean lower electron density.

### Step 4: Compare the Oxidation States
Now, we compare the oxidation states:
- Chromium: 0
- Vanadium: -1
- Titanium: -2
- Manganese: +1

Among these, manganese has the highest oxidation state (+1), which implies it has the least electron density.

**Hint:** The metal with the highest oxidation state will have the weakest bond due to the least electron density available for synergic bonding.

### Step 5: Conclusion
Since manganese has the highest oxidation state and thus the least electron density, it will have the longest M-C bond length. Therefore, the correct match for the highest 'M-C' bond length is in the case of manganese with the complex `[Mn(CO)6]^-1`.

**Final Answer:** The highest 'M-C' bond length is found in the complex with manganese, `[Mn(CO)6]^-1`.