Two new dinuclear mu -CO32- Cu(II) complexes with different coordination modes for the carbonato bridge have been obtained by fixation of atmospheric CO2 and also directly prepared from the carbonate salt. The compounds comprise: [Cu-2(mu -CO3)(dpyam)(4)](ClO4)(2)(H2O)(4) (1), and [Cu-2(mu -CO3)(2)(dpyam)(2)](H2O) (2), (in which dpyam = di-2-pyridylamine). For 1, the carbonate ligand acts as a bridge between two Cu(II) centres showing an anti-anti (mu-eta (1)-eta (1)-CO32-) coordination mode with a distorted square-based pyramidal geometry for each Cu(II) environment. Complex 2 involves the di-mu -CO32- bridge with a novel tridentate mu-eta (1)-eta (2)-CO32- coordination mode. The geometry around each copper atom is distorted square-based pyramidal. Susceptibility measurements for both complexes show a weak to moderately strong antiferromagnetic coupling with J values of -90.4 and -9.9 cm (-1) for 1 and 2, respectively. The tridentate co-ordination mode of the carbonate bridge in 2 has not previously been reported for dinuclear Cu(II) complexes. Also its magnetic behaviour and superexchange pathway are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.