What is the formula of Ruthenium carbonyl?

\(Ru(CO)_5\)

The correct answer is (4), \(Ru(CO)_5\).

Ruthenium carbonyl is an organometallic compound with the chemical formula \(Ru(CO)_5\). It is a colorless, light-sensitive liquid that readily decarbonylates upon standing at room temperature. It is of academic interest as an intermediate for the synthesis of metal carbonyl complexes.

The other answer choices are incorrect. \(Ru(CO)_4\) is not a valid chemical formula. \(Ru(CO)_6\) would have too many carbon monoxide ligands. \(Ru(CO)_8\) would have too many metal atoms.