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 angstrom for coordination number (CN) 6 (9.8%), 0.22 angstrom for CN 7 (8.7%), 0.21 angstrom for CN 8 (8.0%), 0.21 angstrom for CN 9 (8.2%) and 0.18 angstrom 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+.