Molybdenum is the fourth member of the second transition series and is placed with chromium and tungsten in Group 6 of the Periodic Table. In its chemical properties molybdenum resembles tungsten and vanadium, the first member of Group 5, rather than chromium. Chemically molybdenum is versatile, forming compounds in a range of readily interconvertible oxidation states, complexes with many inorganic and organic ligands including physiologically important compounds, binuclear and polynuclear species involving bridging ligands (oxide, hydroxide, sulfide) or direct metal-metal bonds between molybdenum atoms, and compounds in which the molybdenum coordination number ranges from four to eight.