Oxidation state and covalency in f-element metallocenes (M = Ce, Th, Pu): a combined CASSCF and topological study

Abstract

CASSCF calculated wavefunctions are presented for three f-element metallocenes, MCOT2 (M = Ce, Th, Pu; COT = η(8)-C8H8). The configurational admixture of these systems is investigated and, where the ThCOT2 ground state is well-defined as a monodeterminantal Th(IV) state, the cerocene ground state is found to be strong multiconfigurational and to bear strong similarities to that of plutonocene. Associated electronic densities are studied using QTAIM topological analysis and compared to CASSCF-derived densities of the aromatic systems benzene and the COT dianion. This analysis provides evidence of enhanced covalent character in plutonocene, supporting structural data calculated previously. Evidence of charge localisation in found in cerocene, this being most pronounced in its excited state of A(g) symmetry. QTAIM analysis reveals that the ligand electronic structure is very similar in all metallocenes, and density differences show little variation in the ligand between the cerocene ground and excited state. Orbital contributions to integrated QTAIM properties are considered, and excellent agreement with experimentally determined f-orbital occupation is obtained. All methods of analysis support a Ce(IV) or mixed valence assignment of the cerocene ground state, whereas the A(g) excited state is best described as a Ce(III) state.

A Structurally Characterized Organometallic Plutonium(IV) Complex

“…Computations of Pu‐containing molecules are still rather scarce in the literature, mainly because of the number of electrons contained in such systems and the treatment of relativistic/correlation effects. Previous studies33, 34, 35, 36 have shown that the metal−carbon bonds in the highly symmetric plutonocene molecule have a multiconfigurational character with significant contribution of static as well as dynamic electron correlations. In view of the low point symmetry and the considerable dimension of the Pu(1,3‐COT′′)(1,4‐COT′′) molecule, compromises in the selection of basis sets and the chosen methods had to be made.…”

“…Subsequently, anionic complexes of the type K[An(COT) 2 ] (An=Np, Pu) were reported24, 25 and the series of neutral sandwich complexes was extended to a number of ring‐substituted derivatives such as An(EtCOT) 2 , An( n BuCOT) 2 , and An(Me 4 COT) 2 (An=Pa, Np, Pu) 25, 26, 27, 28. From the very beginning until today, the magnetic and electronic properties of these actinocenes and related sandwich complexes intrigued theoretical chemists 29, 30, 31, 32, 33, 34, 35, 36, 37, 38. For example, it was shown that, while 5f contributions to covalency in these actinocenes are smaller in magnitude than 6d contributions, the variation in covalency is almost entirely accounted for by the variation in the 5f contribution 36…”

“…Based on reported computational studies of Pu systems, different computational methods (namely ROHF, MP2, and DFT) were used. For 4 , the ground state was assumed to have the same multiplicity as nonsubstituted plutonocene with a ground state of 5 A g 14, 15, 34, 35, 61…”

Electronic structure theory to decipher the chemical bonding in actinide systems

“…Historically, the first theoretical work on U(COT) 2 including relativistic effects was done by Pyykkö and Lohr in 1981 [100] using relativistically parametrised extended Hückel calculations. Whereas a single-determinant wave function could describe the ground states of thorocene and uranocene [194,195], a multiconfigurational description of the wave function is needed for the later actinides [196,197]. For example, Kerridge and Kaltsoyannis [196] performed all-electron spin-orbit coupling complete active space self-consistent field calculations including dynamic correlation via second order perturbation theory (SOC-CASPT2) for the ground and low-lying excited states of M(COT) 2 (M=Th, U, Pu, Cm).…”

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029

“…[68][69] Many "non-classical" tetravalent cerium molecules have also been prepared, including organometallic compounds, [70][71][72] amide, 65,70,[73][74][75][76][77][78] porphyrins [79][80][81][82][83] and a variety of other multidentate N-donor ligands. [84][85][86][87][88][89][90][91] Among these, the lanthanide sandwich molecule cerocene, (C8H8)2Ce, [92][93][94][95][96][97][98][99][100][101][102][103] has played a central role in development of electronic structure models for tetravalent cerium molecules. Theoretical, magnetic, and L3-edge XANES studies of (C8H8)2Ce resulted in observation of the Kondo effect in a single molecule -a phenomenon that is typically reserved for metals.…”