Bond Valence Sums in Coordination Chemistry Using Oxidation-State-IndependentR0Values. A Simple Method for Calculating the Oxidation State of Iron in Fe−O Complexes

The use of the bond valence sum in Fe−O complexes is discussed using data from the Cambridge Structural Database. The oxidation state of Fe in complexes containing Fe bonded only to O can be calculated by ∑i exp[(R0 − ri)/b], where ri is the observed Fe−Oi bond length, R0 is a constant = 1.745 Å, and b is taken to be 0.37. The R0 value can be viewed as a bond length of unit valence. Deviations from an integer value indicate a problem with the crystal structure or electron delocalization in multinuclear complexes. The oxidation-state-independent R0 value of 1.745 Å was calculated using 297 FeOn complexes with n = 3−8. An R0 value of 1.714 Å for Fe(II) was determined using 74 complexes, and a value of 1.750 Å was determined for Fe(III) using 223 complexes.