KxH1-xNi(OH2)(4)[Ru-2(CO3)(4)]center dot zH(2)O is a ferrimagnet (T-c = 4.3 K) formed from the reaction of K-3[Ru(II/III)2(CO3)(4)] and Ni-II in water. It possesses a new 3-D network structural motif composed of linked chains and mu(3)-CO3 linkages to both Ru and Ni sites. Each Ni(II) bonds to four oxygens and to two [Ru-2(CO3)(4)](3-) moieties in a cis manner, and four mu(3)-CO3 groups from each [Ru-2(CO3)(4)](3-) have two oxygens bonding to the Ru-2 moiety, forming the typical paddle-wheel core, and trans pairs of the third CO32- oxygen axially bonded to either another Ru-2 or Ni-ll.