Are the Ground States of the Later Actinocenes Multiconfigurational? All-Electron Spin−Orbit Coupled CASPT2 Calculations on An(η⁸-C₈H₈)₂(An = Th, U, Pu, Cm)

Abstract

Spin-orbit free and spin-orbit coupled CASPT2 wave functions and energies are presented for the ground and low-lying excited states of four actinide element sandwich molecules; thorocene (ThCOT(2)), uranocene (UCOT(2)), plutonocene (PuCOT(2)), and curocene (CmCOT(2)). Spin-orbit coupling is found to make little difference to the equilibrium geometry of uranocene and plutonocene but has a significant effect on the energy spectrum of all the systems considered here other than thorocene. In all cases, however, the spin-orbit free ground states make the dominant contribution to their spin-orbit coupled counterparts. Following work presented in J. Phys. Chem. A 2009, 113, 2896, the variation in the multiconfigurational character of the ground-state wave functions as the 5f series is crossed is quantified via the occupation of the a(u)/b(1u) (e(2u)) metal-ring bonding and antibonding natural orbitals. The ground state of plutonocene is found to be nondegenerate with |M(J)| = 0, in agreement with its temperature-independent paramagnetism.

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 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…”

Intermediate-Valence Tautomerism in Decamethylytterbocene Complexes of Methyl-Substituted Bipyridines

“…Recent experimental and theoretical studies of certain Ce-and Yb-based organometallic complexes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] have highlighted the importance of multiconfigurational interactions in understanding their electronic, structural, and magnetic properties. The canonical examples include Ce(cot) 2 , also known as "cerocene," where cot = cyclooctatetraene = C 8 H 8 , and Cp * 2 Yb(bipy), where Cp * = pentamethylcyclopentadienyl, and bipy = 2,2 ′ -bipyridine.…”