Explain what is ferrocene?

Ferrocene is an organometallic compound is which an ##Fe^(2+)## nucleus is sandwiched between two cyclopentadienyl rings, ##C_5H_5^-##.

And thus ##"ferrocene"## can be represented by the formula ##Fe(eta^5-C_5H_5)_2##. The cyclopentadienyl ligands sandwich the iron centre such that all the hydrogen atoms are equivalent.

Ferrocene was thus the first example of the sandwich , so called because of its structure, where the metal is the meat in the sandwich. See https://en.wikipedia.org/wiki/Ferrocene

Ferrocene is (i) quite air stable, and (ii) soluble in hexanes. These are unusual properties for an iron complex, and when it appeared in the literature it caused a great deal of excitement and launched organometallic chemistry as an important field of chemistry. Geoffrey Wilikinson, and Ernst Fischer shared the Nobel Prize in 1973 for its discovery.

You will probably make some of this stuff as orange crystals as an inorganic experiment. I always thought it had a rusty iron smell, or maybe rusty iron contains ferrocene.