Haloaryl compounds are lithiated and thereafter metalated with one of the following organometallic groups: Sn(n--Bu).sub.3 or SnMe.sub.3. The resulting aryltin compound can be transmetalated in site-specific reaction with one of the following organometallic groups: HgX, Hg(OAc).sub.2, BX.sub.3, or BZ.sub.2, wherein X is Cl, Br, or I, and Z is alkyl or alkoxy. The metalated compounds are subsequently radiohalogenated via a demetalation reaction. A functional group suitable for conjugation to protein can be added subsequent or preferably prior to the radiohalogenation.Also compounds of the formula: R.sub.1 --Ar--R.sub.2, wherein R.sub.1 is either a radiohalogen or any one of the organometallic groups stated above, Ar is aromatic or heteroaromatic ring, and R.sub.2 is a short-chain substituent that does not activate the aromatic ring and that bears a functional group, or a precursor thereof, suitable for conjugation to protein under conditions that preserve the biological activity of the protein.The radiohalogenated small molecules are conjugated to proteins such as monoclonal antibodies for use in diagnosis and therapy.