Bicarbonate ion reacts with the dinuclear nickel(II) complex containing the taec ligand (taec = N,N',N',N"-tetrakis-(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane) in buffered aqueous solution to form the mu-eta(2),eta(2)-carbonate complex with a large effective binding constant for bicarbonate ion, log K-B = 4.39 at pH = 7.4. In contrast, the dinuclear nickel(II) complex containing the o-xyl-DMC2 ligand (o-xyl-DMC2 = alpha,alpha'-bis(5,7-dimeth-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene) does not react with bicarbonate or carbonate ion in aqueous solution. In propylane carbonate, the reaction of [Ni-2(o-xyl-DMC2)](4+) with bicarbonate proceeds rapidly to form the mu-eta(2),eta(2)-carbonate complex. The structure of this carbonate complex has been determined by an X-ray diffraction study that confirms the mu-eta(2),eta(2)-carbonate binding mode. A mononuclear analogue of [Ni-2(taec)](4+), [Ni(2,3,2-tetraamine)](2+) does not form a detectable mononuclear or dinuclear product with bicarbonate ion in aqueous solution, but [NiDMC](2+) (DMC = 5,7-dimethyl-1,4,8,11-tetraazacyclotetradecane) reacts slowly with carbonate ion in aqueous solution to loan a 2:1 complex.