ON CORRELATING THE FREQUENCY OF THE F-7(0)-]D-5(0) TRANSITION IN EU(3+) COMPLEXES WITH THE SUM OF NEPHELAUXETIC PARAMETERS FOR ALL OF THE COORDINATING ATOMS

The frequency of the F-7(0) --> D-5(0) transition of EU(3+) is found to correlate with the sum of derived nephelauxetic parameters of the ligating atoms in Eu3+ complexes. Parameters for nine distinct coordinating atom types are obtained via a multiple linear regression method applied to experimental F-7(0) --> D-5(0) transition frequencies of twenty-seven 9-coordinate Eu3+ complexes with known ligand constituents. The linear correlation coefficient is 0.99. Addition of data from 7-, 8-, 10-, and 11-coordinate complexes and use of a multiplicative constant, characteristic of each coordination number, produces a second linear correlation covering thirty-seven systems with a correlation coefficient of 0.98. The present approach seeks to understand the factors which influence the F-7(0) --> D-5(0) transition frequency and to make predictions of this quantity for particular ligand environments. It is completely empirical in nature; however, the results may be understood in terms of an underlying theoretical cause. The parameters obtained here correlate roughly with the nephelauxetic h-values of Jorgensen for the few cases where appropriate ligand matches are available. The F-7(0) --> D-5(0) term separation will decrease from its free-ion value of 17 374 cm(-1) with increasing Eu3+-ligand covalency, and the presently derived parameters are consistent with this idea. The correlation presented here is clearly superior to our earlier attempt to identify a relationship between the F-7(0) --> D-5(0) transition frequency and the total formal charge on the ligands in a complex.