Which of the following is $ \pi - $ complex?
A) Trimethylaluminum
B) Ferrocene
C) Diethyl Zinc
D) Nickel Carbonyl

Hint : $ \pi $ -effects or $ \pi $ -interactions are a form of non-covalent interaction involving systems in chemistry. The electron-rich system will interact with a metal (cationic or neutral), an anion, another molecule, and even another system, much as in an electrostatic interaction where a region of negative charge interacts with a positive charge.

Complete Step By Step Answer:
The following are the most common -interactions:
Metal– $ \pi $ interactions: involves the contact of a metal with the face of a $ \pi - $ system. The metal may be a cation (known as cation– $ \pi $ interactions) or neutral (neutral– $ \pi $ interactions).
Polar– $ \pi $ interactions: a polar molecule and a $ \pi - $ system's quadrupole moment interact.
Aromatic–aromatic interactions ( $ \pi $ stacking): involves aromatic molecules interacting with one another.
 $ \pi $ - Donor: Interactions between a low-energy empty orbital (acceptor) and a high-energy filled orbital (donor) are known as donor–acceptor interactions (donor).
Anion- $ \pi $ system interaction: Interactions between anion and other $ \pi $ system.
Cation- $ \pi $ system Interaction: Interactions of a cation with a $ \pi $ -system are called cation– $ \pi $ interactions.
Ferrocene has the formula \[Fe{({C_5}{H_5})_2}\] and is an organometallic compound. Two cyclopentadienyl rings are connected on opposite sides of a central iron atom in the molecule. It's a camphor-like orange solid that sublimes above room temperature and dissolves in most organic solvents.
According to Hückel's law, the number of -electrons on each ring is then six, making it aromatic. Covalent bonding is used to share the twelve -electrons with the metal. The complex achieves an $ 18 $ -electron configuration, which accounts for its stability, since $ F{e^{2 + }} $ has six d-electrons.
The correct option is B.

Note :
Pi ligands are a type of organometallic ligand that includes linear molecules like ethylene and allyl, as well as cyclic molecules like cyclopentadienyl, and have extended pi systems. These molecules have a direct effect on the reactivity of the organometallic complex as a dative L-type ligand.