Which of the following is a `pi` – complex

Options

A. Trimethyl aluminum
B. Ferrocene
C. Diethyl zinc
D. Nickel carbonyl

Answer

To determine which of the given compounds is a pi-complex, we need to analyze each option based on the characteristics of pi-complexes. Pi-complexes are formed when an electron-rich system interacts with a metal, typically involving the donation of electron density from a pi bond or a lone pair of electrons.

### Step-by-Step Solution:

1. **Understanding Pi-Complexes**:
- Pi-complexes involve the interaction of a metal with an electron-rich system, where the electron density from a pi bond is donated to the metal. This can occur with alkenes, alkynes, or aromatic compounds.

2. **Analyzing Trimethylaluminium**:
- Trimethylaluminium (Al(CH3)3) is an organoaluminium compound. It does not have a pi bond or an electron-rich system that can interact with a metal in a pi-complex manner.
- **Conclusion**: Not a pi-complex.

3. **Analyzing Ferrocene**:
- Ferrocene consists of an iron (Fe) atom sandwiched between two cyclopentadienyl anions (C5H5^-). The cyclopentadienyl rings have pi electrons that can interact with the iron atom.
- **Conclusion**: Ferrocene is a pi-complex because it has an electron-rich system (the cyclopentadienyl rings) interacting with the metal (iron).

4. **Analyzing Diethyl Zinc**:
- Diethyl zinc (Zn(C2H5)2) is an organozinc compound that does not contain any pi bonds. It consists of zinc bonded to two ethyl groups, which do not provide the necessary electron-rich system for pi-complex formation.
- **Conclusion**: Not a pi-complex.

5. **Analyzing Nickel Carbonyl**:
- Nickel carbonyl (Ni(CO)4) is a coordination compound where nickel is bonded to four carbonyl (CO) ligands. While CO is a strong field ligand, it does not form a pi-complex in the same way as alkenes or aromatic compounds do.
- **Conclusion**: Not a pi-complex.

### Final Answer:
The only compound that qualifies as a pi-complex among the options given is **Ferrocene**.