As I promised I am now going to show you some mononuclear uranium carbonate complexes.
R. Graziani, G. Bombieri and E. Forsellini, Dalton Transactions, 1972, 2059-2061 report the structure of tetramethyl ammonium salt of the [UO2(CO3)3]4- anion. This is a mononuclear uranyl carbonate complex. A similar complex was observed in the crystal structure of hydrated calcium uranyl carbonate (Liebigite) by K. Mereiter. Below I am showing two pictures of the uranyl carbonate complex from the Dalton paper.