Bond-length distributions for ions bonded to oxygen: results for the lanthanides and actinides and discussion of the f-block contraction

Bond-length distributions have been examined for 84 configurations of the lanthanide ions and 22 configurations of the actinide ions bonded to oxygen, for 1317 coordination polyhedra and 10 700 bond distances for the lanthanide ions, and 671 coordination polyhedra and 4754 bond distances for the actinide ions. A linear correlation between mean bond length and coordination number is observed for the trivalent lanthanides ions bonded to O²⁻. The lanthanide contraction for the trivalent lanthanide ions bonded to O²⁻ is shown to vary as a function of coordination number, and to diminish in scale with an increasing coordination number. The decrease in mean bond length from La³⁺ to Lu³⁺ is 0.25 Å for coordination number (CN) 6 (9.8%), 0.22 Å for CN 7 (8.7%), 0.21 Å for CN 8 (8.0%), 0.21 Å for CN 9 (8.2%) and 0.18 Å for CN 10 (6.9%). The crystal chemistry of Np⁵⁺ and Np⁶⁺ is shown to be very similar to that of U⁶⁺ when bonded to O²⁻, but differs for Np⁷⁺.