Within the scope of this present work the exploitation of the coordination modes of the squarate and also its acid properties were envisaged. In a systematic way, using either a coordinative or a hydrogen-bonding motif, three classes of compounds have been synthesized and characterized by single-crystal X-ray diffraction: 1. Hydrogen-bonded systems based on acid-base reactions of biguanide derivatives (N,N-dimethylbiguanide, N-phenylbiguanide, N-o-tolylbiguanide) and squaric acid. It is envisaged here the correlation between the pH of the media, the p-delocalization and hydrogen bond lengths. The shortest hydrogen bonds are present in very strong acidic media, due to the presence of the [SQH]- ([SQH]- = monohydrogensquarate) together with [BigH3]2+ ([BigH3]2+= biguanidinium). In these ions the p-delocalization is less pronounced than in [SQ]2- ([SQ]2- = squarate dianion) and [BigH2]+, ([BigH2]+ = monohydrogenbiguanidinium) which are present in basic media.2. Squarate complexes of transition metals. A dinuclear Cu(II)-squarate compound and a series of homo- and heteropolynuclear compounds with magnetically interesting cations have been obtained. The coordination modes of the squarate dianion are µ-1,2-bis(monodentate) in the dinuclear compound and µ-1,3-bis(monodentate) in polynuclear compounds.3. Ag(I)-squarate coordination polymers which combine the bridging ability of squarate with the flexibility in the coordination number, donor type and the geometry adopted by the AgI cation.