Bond valence sums in coordination chemistry. Calculation of the oxidation state of chromium in complexes containing only Cr-O bonds and a redetermination of the crystal structure of potassium tetra(peroxo)chromate(V)

A simple method for calculating the oxidation state of Cr in complexes cont aining only Cr-O bonds is presented. A total of 242 CrOn, fragments with n = 3-6 were retrieved from the Cambridge Structural Database (CSD) and, toge ther with the data for K3CrO8, were analyzed using the bond valence sum met hod. New R-0 values for Cr(II) of 1.739(21) Angstrom, Cr(III) of 1.708(7) A ngstrom, Cr(V) of 1.762(14) Angstrom, and Cr(VI) of 1.793(7) Angstrom were derived. An average Ro value of 1.724 Angstrom for Cr-O reproduces the oxid ation state of 96 of the 110 Cr(II), Cr(III), and Cr(IV) CrOn complexes (n 3-6) and that of K3CrO8 within 0.30 valence units. The crystal structure of K3CrO8 was redetermined at 173 K to provide accurate data for a Cr complex with both high oxidation state and coordination number. Potassium tetraper oxochromate(V), K3CrO8, is tetragonal, Space group (I) over bar 42m, a = b = 6.6940(3) Angstrom, c = 7.7536(5) Angstrom, Z = 2. The difficulties with fitting the observed valence for Cr(V) and Cr(VI) complexes with coordinati on numbers 4 and 5 are discussed. The use of bond valence sums in gaining c hemical insight into Cr complexes with noninnocent ligands and in establish ing oxidation states in Cr clusters is presented. An analysis of the Cr-O b ond distances used in the calculations shows a large range of values that c an be understood in terms of the bond valence sum calculation.