Tetracarbonylmolybdenum and halotricarbonylrhenium complexes of laterally extendable or laterally extended bipyridines such as 5,5'-dibromo-2,2'-bipyridine, 5,5'-bis(trimethylsilylethynyl)-2,2'-bipyridine, or 3,8-dibromophenanthroline have been prepared. These complexes are potential precursors for covalent T-joints by substitution of carbon monoxide in the molybdenum complexes or halide in the rhenium complexes by linear terminal ligands. The formation of a covalent T-joint has been demonstrated by attachment of Mo(CO) 3 (5,5'-bis(trimethylsilylethynyl)-2,2'-bipyridine) units to the free isocyanide groups of ReCl(CO) 3 (CNC 6 H 1 0 CN) 2 .