Novel labeling reagents are useful for radiolabeling proteins such as antibodies with radionuclide metals such as .sup.99m Tc, .sup.186 Re, and .sup.188 Re. The reagents are of the following formula: ##STR1## wherein: each X is independently chosen from O, S, NH, or NR.sub.2, wherein R.sub.2 is a protecting group or an electron withdrawing group;R.sub.1 represents a protecting group, wherein each R.sub.1 is a separate protecting group or the two R.sub.1 symbols are taken together to represent a single protecting group; andeach T is independently chosen from hydrogen, lower alkyl groups of from 1 to about 6 carbon atoms, electron withdrawing groups, and lower alkyl groups of from 1 to about 6 carbon atoms substituted with electron withdrawing group(s).