Molybdenum is the only 4d element with a biological function, this is probably connected with its bioavailability since it is soluble in seawater as molybdate(VI), MoO₄^2-. The physiologically active oxidation states of molybdenum are from +IV to +VI. The compounds found under these conditions involve O and S ligands, and an important biological function is to catalyses the transfer of oxygen to a two electron substrate.

⁹⁵Mo is 15.9% abundant with a spin of 5/2 and is 9.6% abundant with a spin of 5/2

The remaining isotopes ^{92,94,96,98,100}Mo, do not have a nuclear spin, giving molbdenum compounds a very characteristic 'finger print' with a strong central line of around 75% of the intensity and a set of six satellites. The hyperfine interaction varies with the ligands attached to the molybdenum, but is often in the 4-5 mT region, the small difference between the magnetic moments of the two spin active+ isotopes means that the isotope splittings are rarely observed. Mo^V is a d¹ ion, Mo^I is a d⁵ ion.