Efficient electrochemical syntheses of "homocoenzyme B(12)" (2, Co(beta)-(5'-deoxy-5'-adenosyl-methyl)-cob(III)alamin) and "bishomocoenzyme B(12)" (3, Co(beta)-[2-(5'-deoxy-5'-adenosyl)-ethyl]-cob(III)alamin) are reported here. These syntheses have provided crystalline samples of 2 and 3 in 94 and 77 % yield, respectively. In addition, in-depth investigations of the structures of 2 and 3 in solution were carried out and a high-resolution crystal structure of 2 was obtained. The two homologues of coenzyme B(12) (2 and 3) are suggested to function as covalent structural mimics of the hypothetical enzyme-bound "activated" (that is, "stretched" or even homolytically cleaved) states of the B(12) cofactor. From crude molecular models, the crucial distances from the corrin-bound cobalt center to the C5' atom of the (homo)adenosine moieties in 2 and 3 were estimated to be about 3.0 and 4.4 A, respectively. These values are roughly the same as those found in the two "activated" forms of coenzyme B(12) in the crystal structure of glutamate mutase. Indeed, in the crystal structure of 2, the cobalt center was observed to be at a distance of 2.99 A from the C5' atom of the homoadenosine moiety and the latter was found to be present in the unusual syn conformation. In solution, the organometallic moieties of 2 and 3 were shown to be rather flexible and to be considerably more dynamic than the equivalent group in coenzyme B(12). The homoadenosine moiety of 2 was indicated to occur in both the syn and the anti conformations.