Assignment of oxidation states in metal complexes. Cerium(III) or cerium(IV) and other questions

The use of the bond valence sum to assign correctly the oxidation state of a metal ion in a complex is discussed. Cerium complexes are used as examples since the oxidation state of Ce has been incorrectly assigned in a surprising number of publications. The recommended R0 values for Ce(III)–O of 2.118 Å, Ce(IV)–O of 2.070 Å, Ce(III)–N of 2.251 Å, and for Ce(IV)–N of 2.202 Å were derived from analyses of homoleptic Ce–O, Ce–N, and heteroleptic Ce–O and –N complexes. These R0 values can be used to assign correctly the oxidation state of Ce in complexes containing any combination of Ce–O or Ce–N bonds. An incorrect oxidation state assignment usually arises when the oxidation state of Ce or Pr in the product is assumed to be same as that of the starting Ce or Pr compound, but an oxidation or reduction has occurred. Problems with two related Sn complexes may have arisen because of a mix up in the starting materials.