The reaction of K(2)TzC (H(2)TzC = tetrazole-5-carboxylic acid) and either MnCl(2) x 4 H(2)O or CuCl(2) x 2 H(2)O under hydrothermal conditions leads to the formation of three new polynuclear complexes of formula [Mn(TzC)(H(2)O)(2)](n) 1 and [M(2)(TzC)(2)(H(2)O)(6)] x nH(2)O (M = Mn(II) 2, n = 4 and Cu(II) 3, n = 2). Complex 1 displays a 1D planar zig-zag chain structure where the ligand is disorderd into two positions and exhibits two mu(3)-chelating/bridging tetradentate coordination modes, one through the two carboxylate oxygen atoms and the neighbouring nitrogen atom of the tetrazolate ring and the other one through three neighboring nitrogen atoms belonging to the tetrazole ring and one of the carboxylate oxygen atoms. The structure of 2 and 3 consists of centrosymmetric dinuclear molecules, in which the ligands exhibit a mu(2)-chelating/bridging coordination mode through one of the carboxylate oxygen atoms and two neighbouring nitrogen atoms of the tetrazolate ring, generating a planar M(N-N)(2)M hexagon. Variable-temperature magnetic susceptibility studies reveal a weak antiferromagnetic interaction for 1 (J = -1.5 cm(-1)). In spite of the coplanarity of the tetrazolate and metal coordination planes in 2 and 3, they exhibit very weak antiferromagnetic interactions. DFT calculations have been performed in order to explain this unexpected magnetic behaviour.