Which of the following pairs is not correctly matched

Options

A. Effective atomic nmber of Pt in `[PtCl_(6)]^(2-)= 84`
B. Absorption peak for `[Cr^(III)(NH_(3))_(6) ]^(+3) = 21680cm^(-1)`
C. Crystal field stabilization energy of `d^(2)` in weak ligand field `= ( - ) 0.8 Delta _(0)`
D. Example of weak ligand field for `d^(5)` configuration `=[ Mn^(II) F_(6)]^(-4)`

Explanation

To determine which of the given pairs is not correctly matched, we need to analyze each option based on the principles of coordination chemistry, particularly focusing on effective atomic numbers, crystal field theory, and ligand field strength.

### Step-by-Step Solution:

1. **Understanding Effective Atomic Number (EAN) for PtCl6^2-**:
- The effective atomic number (EAN) can be calculated using the formula:
\[ \text{EAN} = Z + n - x \]
where \( Z \) is the atomic number of the metal, \( n \) is the number of electrons contributed by the ligands, and \( x \) is the charge on the complex.
- For platinum (Pt), the atomic number \( Z \) is 78. In PtCl6^2-, there are 6 chloride ions contributing 6 electrons, and the charge of the complex is -2.
- Therefore, the calculation is:
\[ \text{EAN} = 78 + 6 - 2 = 82 \]
- The statement claims the EAN is 84, which is incorrect.

2. **Evaluating the Adsorption Peak of [Cr(NH3)6]3+**:
- The given value of the adsorption peak for Cr^3+ in an octahedral complex with ammonia (a strong field ligand) is stated as 21,680 cm^-1.
- This value is consistent with known data for such complexes, thus this option is correct.

3. **Crystal Field Stabilizing Energy (CFSE) for d^2 in a Weak Field**:
- For a d^2 configuration in a weak field, the electrons will occupy the lower energy T2g orbitals.
- The CFSE can be calculated as:
\[ \text{CFSE} = n_{T2g} \times (-0.4 \Delta_o) + n_{Eg} \times (0.6 \Delta_o) \]
where \( n_{T2g} \) is the number of electrons in T2g and \( n_{Eg} \) is the number of electrons in Eg.
- For d^2, both electrons will be in T2g:
\[ \text{CFSE} = 2 \times (-0.4 \Delta_o) = -0.8 \Delta_o \]
- This statement is correct.

4. **Identifying Weak Ligand Field for d^5 Configuration**:
- A d^5 configuration can be found in Mn^2+ (which has a d^5 configuration).
- In a weak field, the electrons will remain unpaired in the T2g and Eg orbitals.
- This statement is also correct.

### Conclusion:
The only incorrect statement is regarding the effective atomic number of platinum in PtCl6^2-, which was stated to be 84, but the correct calculation gives us 82.

### Final Answer:
The pair that is not correctly matched is: **Effective atomic number of platinum in PtCl6^2- is 84**.