CO2 and HCO3- react with the dinuclear hydroxo-complex [Ni(mcN3)(mu-OH)]2(PF6)2 (mcN3 = 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene) to form micro-CO3 bridged nickel(II) complexes, [{Ni(mcN3)}2(mu-CO3)](PF6)2 (1a) with a symmetric core in which both nickel atoms are five-coordinate and [Ni(mcN3)(mu-CO3)Ni(mcN3)(MeCN)](PF6)2 (1b) with an asymmetric dinuclear core containing five- and six-coordinate nickel atoms. The magnetic behaviour indicates the existence of antiferromagnetic coupling between the metallic centres. A substantial increase in the value of J occurs when the symmetric five-coordinate nickel species transforms to an asymmetric five- and six-coordinate species by axial coordination of acetonitrile.