COMPLEX ALKALIMETAL-CARBONATES-HYDROGENCA RBONATES WITH PENTACOORDINATED ZINC - NAA(2)(ZN-2[H(CO3)(2)](CO3)(2)(H2O)(2)) WITH A=K, RB

The new tal-hydrogenbis(carbonato)-bis(carbonato)complexes with penta- coordinated zinc atoms, NaA(2)(Zn-2[H(CO3)(2)] (CO3)(2)(H2O)(2)) With A = K or Rb, crystallize in the monoclinic system, P2(1)/n (no. 14), Z = 2 with a = 1057,9(1), b = 1102,2(1), c = 514,9(1) pm, beta = 92,94( 1)degrees, V-EZ = 599,6(2) . 10(6) pm(3), R = 0,052, R(w) = 0,029 for the potassium compound and with a = 1068,2(2), b = 1131,6(4), c = 518, 6(1) pm, beta = 92,07(1)degrees V-EZ = 626,4(4). 10(6) pm(3), R = 0,05 0, R(w) = 0,028 for the rubidium compound. These compounds are the fir st to be structurally characterized with a simultaneous presence of di screte [H(CO3)(2)](3-) and CO32- anions. The crystal structure of the isostructural compounds contains two-dimensional infinite (Zn-2[H(CO3) (2)] (CO3)(2)(H2O)(2)) layers parallel [101] with zinc coordinated in a slightly distorted trigonal-bipyramidal geometry. The layers are hel d together by hydrogen bonds from water molecules to oxygen atoms of t he carbonate groups in adjacent layers. Potassium or rubidium atoms ar e located between the layers, and sodium atoms in smaller vacancies wi thin the layers. The hydrogen bond of the [H(CO3)(2)](3-) anion is ver y short, O ... O: 247,4(2) pm (K) and 246,9(5) pm (Rb). The carbonate groups in both compounds exhibit significant deviations from D-3h symm etry in accordance with the vibrational modes observed in the IR spect ra. The thermal behaviour of the potassium compound is also reported.