ELECTRONIC-STRUCTURE AND BONDING IN THE HOPPE ANIONS [FE2O5](6-) AND [CO2O4](4-)

A molecular orbital study of the Hoppe anions [Fe2O5](6-) and [Co2O4]( 4-) reveals the existence of metal-oxygen pi bonding in the plane of t he molecule. In [Fe2O5](6-) the double-bond character is delocalized t hroughout the terminal and bridging Fe-O bonds, whereas in [Co2O4](4-) only the terminal Co-O bonds have double-bond character Taking into a ccount the existence of such pi bonds, these anions can be considered as sixteen-electron species. Both anions are seen to be susceptible to second-order Jahn-Teller distortions, The relative position of the al kali cations is related to the orientation of the high-lying lone-pair orbitals, and the cation anion interactions strengthen the M-O and Co -Co bonds.