One currently preferred diagnostic composition within the scope of the present invention includes derivatives of the neutral metal cluster Mn12 X12 (OYR)16 (L)4, where OYR is an oxyacid such asbenzoic acid, acetic acid, methyl sulphonic acid, methyl phosphonic acid; Lis a neutral donor such as water, alcohol, pyridine, or other amines; and Xis a chalcogen, such as O or S. A specific example of the