The present invention relates to compounds and related technetium and rhenium complexes thereof which are suitable for imaging or therapeutic treatment of tumors, e.g., carcinomas, melanomas and other tumors. In another embodiment, the invention relates to methods of imaging tumors using radiolabeled metal complexes. Preferred radiolabeled complexes for imaging tumors include technetium and rhenium complexes. The high tumor uptake and significant tumor/nontumor ratios of the technetium complexes of the invention indicate that such small technetium-99m-based molecular probes can be developed as in-vivo diagnostic agents for melanoma and its metastases. In yet another embodiment, the invention relates to methods of treatment of tumors using a radiolabeled metal complex as a radiopharmaceutical agent to treat the tumor.