The crystal and molecular structure of two isomorphous mixed formates (Cu_xM_1-x(HCOO)₂(H₂O)_1.33, M = Mg (1), Co (2), x = 0.74) are presented. The cationic sites are occupied by a mixture of divalent cations (Cu,M)) and the structural motive is composed of double layers of aqua-less, Cu rich polyhedra separated by single layers of M rich ones containing both formate ions and coordinated water molecules. The infrared spectra of isotopically dilute (matrix-isolated HDO molecules) formates are analyzed in the region of the OD stretching modes. A discussion of the different strength of hydrogen bonds present in the structure is made at the light of the influence that the metal ions exert both onto the hydrogen bond donors, through direct coordination (synergetic effect) as well as onto hydrogen bond acceptor strength of the formate oxygens through the M–OCHO interactions.