Creation of an unexpected plane of enhanced covalency in cerium(III) and berkelium(III) terpyridyl complexes

Studying how the ligand design influences the bonding of f-block complexes is crucial to control their properties. Here, the authors report the preparation of Bk(III) and Ce(III) complexes featuring a terpyridyl ligand; structural, spectroscopic, electrochemical, and theoretical analysis reveal that the ligand induces unusual bonding by creating a plane of enhanced bond covalency. Controlling the properties of heavy element complexes, such as those containing berkelium, is challenging because relativistic effects, spin-orbit and ligand-field splitting, and complex metal-ligand bonding, all dictate the final electronic states of the molecules. While the first two of these are currently beyond experimental control, covalent M-L interactions could theoretically be boosted through the employment of chelators with large polarizabilities that substantially shift the electron density in the molecules. This theory is tested by ligating Bk-III with 4'-(4-nitrophenyl)-2,2':6',2"-terpyridine (terpy*), a ligand with a large dipole. The resultant complex, Bk(terpy*)(NO3)(3)(H2O)center dot THF, is benchmarked with its closest electrochemical analog, Ce(terpy*)(NO3)(3)(H2O)center dot THF. Here, we show that enhanced Bk-N interactions with terpy* are observed as predicted. Unexpectedly, induced polarization by terpy* also creates a plane in the molecules wherein the M-L bonds trans to terpy* are shorter than anticipated. Moreover, these molecules are highly anisotropic and rhombic EPR spectra for the Ce-III complex are reported.