title: Trends in covalency for d- and f-element metallocene dichlorides identified using chlorine K-edge X-ray absorption spectroscopy and time-dependent density functional theory.
abstract: We describe the use of Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT) to probe the electronic structure and determine the degree of orbital mixing in M-Cl bonds for (C(5)Me(5))(2)MCl(2) (M = Ti, 1; Zr, 2; Hf, 3; Th, 4; U, 5), where we can directly compare a class of structurally similar compounds for d- and f-elements. Pre-edge features in the Cl K-edge XAS data for the group IV transition-metals 1-3 provide direct evidence of covalent M-Cl orbital mixing. The amount of Cl 3p character was experimentally determined to be 25%, 23%, and 22% per M-Cl bond for 1-3, respectively. For actinides, we find a pre-edge shoulder for 4 (Th) and distinct and weak pre-edge features for U, 5. The amount of Cl 3p character was determined to be 9% for 5, and we were unable to make an experimental determination for 4. Using hybrid DFT calculations with relativistic effective core potentials, the electronic structures of 1-5 were calculated and used as a guide to interpret the experimental Cl K-edge XAS data. For transition-metal compounds 1-3, the pre-edge features arise due to transitions from Cl 1s electrons into the 3d-, 4d-, and 5d-orbitals, with assignments provided in the text. For Th, 4, we find that 5f- and 6d-orbitals are nearly degenerate and give rise to a single pre-edge shoulder in the XAS. For U, 5, we find the 5f- and 6d-orbitals fall into two distinct energy groupings, and Cl K-edge XAS data are interpreted in terms of Cl 1s transitions into both 5f- and 6d-orbitals. Time-dependent DFT was used to calculate the energies and intensities of Cl 1s transitions into empty metal-based orbitals containing Cl 3p character and provide simulated Cl K-edge XAS spectra for 1-4. For 5, which has two unpaired 5f electrons, simulated spectra were obtained from transition dipole calculations using ground-state Kohn-Sham orbitals. To the best of our knowledge, this represents the first application of Cl K-edge XAS to actinide systems. Overall, this study allows trends in orbital mixing within a well-characterized structural motif to be identified and compared between transition-metals and actinide elements. These results show that the orbital mixing for the d-block compounds slightly decreases in covalency with increasing principal quantum number, in the order Ti > Zr approximately = Hf, and that uranium displays approximately half the covalent orbital mixing of transition elements.
authors.affiliation: Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
authors.forename: Stosh A
authors.initials: SA
authors.lastname: Kozimor
authors.forename: Ping
authors.initials: P
authors.lastname: Yang
authors.forename: Enrique R
authors.initials: ER
authors.lastname: Batista
authors.forename: Kevin S
authors.initials: KS
authors.lastname: Boland
authors.forename: Carol J
authors.initials: CJ
authors.lastname: Burns
authors.forename: David L
authors.initials: DL
authors.lastname: Clark
authors.forename: Steven D
authors.initials: SD
authors.lastname: Conradson
authors.forename: Richard L
authors.initials: RL
authors.lastname: Martin
authors.forename: Marianne P
authors.initials: MP
authors.lastname: Wilkerson
authors.forename: Laura E
authors.initials: LE
authors.lastname: Wolfsberg