The trinuclear compound [Ni 2 (µ-CO 3 )(dmpd) 4 (H 2 O)][Ni(dmpd) 2 (H 2 O) 2 ][ClO 4 ] 4 ·H 2 O was obtained from the reaction of basic solutions of nickel(II) perchlorate and 2,2-dimethylpropane-1,3-diamine (dmpd) with atmospheric CO 2. It crystallises in the orthorhombic system, space group Pcab, with a = 18.634(10), b = 25.447(8), c = 25.598(4) Å, Z = 8, R = 0.0437. The three nickel atoms show octahedral co-ordination with three different environments, Ni(CO 3 -O,O′)(dmpd) 2 , Ni(CO 3 -O″)(dmpd) 2 (H 2 O) and Ni(dmpd) 2 (H 2 O) 2 . The carbonate anion acts as a bridge between two nickel ions whereas the [Ni(dmpd) 2 (H 2 O) 2 ] 2+ subunit is linked by hydrogen bonds to the dinuclear group. The dinuclear [Ni 2 (µ-CO 3 )(dmpd) 4 (H 2 O)] 2+ subunit shows a moderately weak antiferromagnetic coupling with a J value of -7.8 cm -1 . The tridentate co-ordination mode found in the carbonate ligand has not previously been reported for nickel. Its magnetic behaviour is discussed.