Extensive ab initio calculations have been carried out to study the structure and bonding characteristics of UO2++,UO2,UO2+,UO2-,OUCO,O2U(CO)(2), and UO2CO3 using various methods including complete active space multiconfiguration self consistent filed (CASMCSCF) and multireference singles plus doubles configuration (MRSDCI) techniques. The calculated vibrational frequencies of these species are compared with the matrix spectra of Andrews and coworkers. The nature of bonding in these species is discussed. (C) 2002 Published by Elsevier Science B.V.