Which of the following statements is incorrect ?

Options

A. According to Werner's coordination theory, secondary valencies of central metal atom is fixed towards a particular ligand.
B. `C_(2)H_(4) and NO` act as non-classical ligand.
C. The bond order of `O_(2)` is 2 and it is colourless gas and paramagnetic in nature.
D. In `PF_(3)`, The P-F bond order decreses when `PF_(3)` forms, complex with a metal and shows synergic bonding.

Answer

To determine which of the given statements is incorrect, let's analyze each statement one by one:

### Step 1: Analyze the First Statement
**Statement 1:** "According to Werner's coordination theory, the secondary valency of the central metal ion is fixed towards a particular ligand."

- **Explanation:** Werner's theory distinguishes between primary and secondary valencies. The primary valency corresponds to the oxidation state of the metal, while the secondary valency refers to the coordination number and the arrangement of ligands around the metal ion. This statement is true.

### Step 2: Analyze the Second Statement
**Statement 2:** "C2H4 and NNU act as non-classical ligands."

- **Explanation:** Non-classical ligands can act as sigma donors and pi acceptors. Ethylene (C2H4) and NNU (which likely refers to a nitrogen-containing compound) can indeed function in this manner. This statement is also true.

### Step 3: Analyze the Third Statement
**Statement 3:** "The bond order of O2 is 2."

- **Explanation:** The bond order of O2 can be calculated using molecular orbital theory, which shows that there are two bonding electrons and no antibonding electrons, resulting in a bond order of 2. This statement is true.

### Step 4: Analyze the Fourth Statement
**Statement 4:** "The bond order of PF3 decreases when PF3 forms a complex with a metal, showing synergic bonding."

- **Explanation:** When PF3 forms a complex with a metal, it primarily exhibits back-bonding rather than synergic bonding. Back-bonding involves the donation of electron density from the filled p-orbitals of the ligands to the empty d-orbitals of the metal, which can lead to a decrease in bond order due to the partial double bond character. However, the term "synergic bonding" is not applicable in this context, making this statement incorrect.

### Conclusion
The incorrect statement is **Statement 4**: "The bond order of PF3 decreases when PF3 forms a complex with a metal, showing synergic bonding."

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