Complex sentence: Iron carbonyl is stable as iron pentacarbonyl, where five carbon monoxide molecules are pendantly bonded to the iron atom, while nickel carbonyl is stable as nickel tetracarbonyl, which has four carbon monoxide molecules pendantly bonded to the nickel atom.
Split into simpler sentences: Iron carbonyl is stable as iron pentacarbonyl, where 5 carbon monoxide molecules are pendantly bonded to the iron atom. Nickel carbonyl is stable as nickel tetracarbonyl, where 4 carbon monoxide molecules are pendantly bonded to the nickel atom.