Are the Ground States of the Later Actinocenes Multiconfigurational? All-Electron Spin-Orbit Coupled CASPT2 Calculations on An(η 8 -C 8 H 8 ) 2 (An ) Th, U, Pu, Cm) Andrew Kerridge and Nikolas Kaltsoyannis* Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, U.K. ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: June 15, 2009 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. 1. Introduction The lanthanocenes and actinocenes MCOT 2 (M ) f element; COT ) η 8 -C 8 H 8 ) are unique in organometallic chemistry in that only for these compounds is a structure of D 8h symmetry found, i.e., the metal center is sandwiched by planar and parallel carbocyclic rings in an eclipsed orienta- tion. To date, Ce-, 1 Th-, 2 U-, 3 Np-, 4 and Pu-containing compounds 4 have been synthesized, along with PaTMCOT 2 (TMCOT ) η 8 -C 8 H 4 (CH 3 ) 4 ), 5 and data on their electronic, molecular, vibrational, and magnetic structure have been obtained. 1-14 Of these data, perhaps the most interesting are the magnetic properties of Ce- and PuCOT 2 , both of which appear to display temperature-independent paramagnetism (TIP) at low temperatures. 12,14 In the former, the TIP has been considered in terms of a molecular Kondo-like effect. 15-17 Such an explanation requires an open-shell singlet electronic structure, leading to suggestions that the cerium ion may exist in a trivalent state, by contrast to its actinide analogue, thorium, which is clearly tetravalent in ThCOT 2 . Although it is not universally accepted, 18 there is both experimental and theoretical evidence to support this view. 13,14,19-21 More specifically, previous computational studies from Dolg et al. concluded that the 1 A 1g ground-state wave function of cerocene is multiconfigurational, with a dominant (∼80%) ring π 3 f δ 1 (Ce(III)) open-shell singlet configuration, and subsequent X-ray absorption spectroscopy experiments appear to support this trivalent picture. 13,14 Very recently, we performed spin-orbit coupled complete active space self-consistent field calculations with dynamic correlation included via second order perturbation theory (hereby referred to as SOC-CASPT2) on MCOT 2 (M ) Th, Pa, Ce), 22 and while our results for the actinide containing species were in excellent agreement with previous work, our data for cerocene were rather different from those of Dolg et al. We concluded that interpretations of the ground-state wave function in terms of the canonical CASSCF configurations is problematic, as the configurational admixture is not stable with respect to the computational methodology employed (specifically, the number of states included in the state averaging procedure). That said, we agreed with Dolg et al. that the cerocene ground state is multiconfigurational but preferred to use the occupation numbers of the natural orbitals as a quantitative measure of this effect. We found a total 4f density (n f ) of 0.90 ( 0.04, in excellent agreement with the experimentally determined value of 0.89 ( 0.03, 14 but this density is not due to the single occupation of a localized 4f orbital, as would be expected for a formally Ce(III) compound. Rather, it is due to significant partial occupation of orbitals exhibiting strong π(e 2u )/f δ covalency, and we suggested that the experimental conclusion of trivalent Ce is a function of the method measuring an effective, rather than a formal, oxidation state. We concluded that if we make the simple distinction that a genuinely Ce(III) compound would be expected to have a metal-localized f electron, while a Ce(IV) compound would not, then cerocene fits better into the latter category, notwithstanding its significant f density. It is well-known that the chemistry of the later actinides is rather different from that of the early members of the series, with the variable valence of the latter being replaced by a dominant lanthanide-like trivalent oxidation state. 23 Dolg and Fulde 24 have previously suggested that the electronic structures of the actinocenes should therefore change from single- configurational An(IV) behavior, as exemplified by thorocene (ThCOT 2 ), to become more cerocene-like as the middle of the series is approached. In the present contribution, we extend our earlier work to three more actinocenes, uranocence (UCOT 2 ), plutonocene (PuCOT 2 ), and curocene (CmCOT 2 ), to probe this suggestion, particularly in the light of our recent conclusions concerning cerocene. For completeness, we also present a summary of our results for ThCOT 2 from reference 22. 2. Computational Details All calculations were performed using the MOLCAS 6.4 code. 25 Spin-orbit free (SOF-) and spin-orbit coupled (SOC-)- CASPT2 calculations were performed using all-electron ANO * To whom correspondence should be addressed. E-mail: n.kaltsoyannis@ ucl.ac.uk. J. Phys. Chem. A 2009, 113, 8737–8745 8737 10.1021/jp903912q CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009