Electronic structure and bonding in cerium (nitride) compounds: Trivalent versus tetravalent cerium

The assignment of the cerium oxidation state in the recently prepared and s tructurally characterized Ce2MnN3 phase is problematic. Based upon EO value s from aqueous solution, one would suggest the crystal composition to be (C e3+)(2)Mn3+(N3-)(3). The experimental data, however, indicate that Ce2MnN3 is nonmagnetic; this alludes to (but does not prove) a cerium oxidation sta te of +4, giving (Ce4+)(2)Mn+(N3-)(3). Previous theoretical work supports t he latter oxidation state assignment. Here we carry out a systematic theore tical examination (using the TB-LMTO-ASA method) of the electronic structur es of a number of Ce compounds, in which the oxidation state on Ce is more easily assigned. The Ce3+ compounds examined are CeF3, CeCl3, CeBr3, and Ce N. Our Ce4+ data set includes CeC, CeO2, Li2CeN2, and Li-2-CeP2. CeN, which is nonmagnetic despite the fact that it contains Ce3+, makes it clear that the absence of a magnetic moment in a compound does not necessarily preclu de the presence of Ce3+. Our calculations indicate that the remaining Ce-ce ntered electron in CeN has substantial d character and is delocalized to fo rm a partial Ce-Ce bond instead of being highly localized in Ce-centered f states, like the unpaired electron in CeBr3. Careful inspection of the band structures of these compounds shows some qualitative trends that may be us eful in assigning oxidation states: in Ce3+ compounds there is at least one occupied band that is clearly made up mostly of Ce states, while in system s with Ce4+, the small Ce contributions are delocalized across many occupie d bands. Using this criterion, we conclude that (Ce4+)(2)Mn+(N3-)(3) is the correct oxidation state assignment for Ce2MnN3. We suspect that the high o xidation state on Ce is stabilized by the large number of N3- ions surround ing it in the crystal.