Which of the following is complex?

The correct answer is option (2) Ferrocene.

A metal complex is a compound that consists of a metal atom or ion bonded to a group of molecules or ions called ligands. The ligands can be neutral or charged, and they can be organic or inorganic.

Ferrocene is a metal complex that consists of an iron atom bonded to two cyclopentadienyl rings. The cyclopentadienyl rings are organic ligands, and they are electron-rich. The iron atom is electron-deficient, so it forms a dative bond with each cyclopentadienyl ring.

The other compounds in the problem are not metal complexes. Trimethyl aluminum is an organometallic compound, but it does not contain a metal-ligand bond. Diethyl zinc is an organometallic compound, but it does not contain a metal-ligand bond. Nickel carbonyl is a coordination compound, but it does not contain a metal-ligand bond.

Therefore, the only metal complex in the problem is ferrocene.