Understanding Structure and Bonding in Early Actinide 6d 0 5f 0 MX 6 q (M ) Th-Np; X ) H, F) Complexes in Comparison with Their Transition Metal 5d 0 Analogues Michal Straka,* ,²,§ Peter Hroba ´ rik, ‡ and Martin Kaupp* ,² Contribution from the Institut fu ¨r Anorganische Chemie, UniVersita ¨t Wu ¨rzburg, Am Hubland, D-97074 Wu ¨rzburg, Germany, and Institute of Inorganic Chemistry, SloVak Academy of Sciences, Du ´ braVska ´ cesta 9, SK-84536 BratislaVa, SloVakia Received August 19, 2004; E-mail: straka@mail.uni-wuerzburg.de; kaupp@mail.uni-wuerzburg.de Abstract: The relationship between structure and bonding in actinide 6d 0 5f 0 MX6 q complexes (M ) Th, Pa, U, Np; X ) H, F; q )-2,-1, 0, +1) has been studied, based on density functional calculations with accurate relativistic actinide pseudopotentials. The detailed comparison of these prototype systems with their 5d 0 transition metal analogues (M ) Hf, Ta, W, Re) reveals in detail how the 5f orbitals modify the structural preferences of the actinide complexes relative to the transition metal systems. Natural bond orbital analyses on the hydride complexes indicate that 5f orbital involvement in σ-bonding favors classical structures based on the octahedron, while d orbital contributions to σ-bonding favor symmetry lowering. The respective roles of f and d orbitals are reversed in the case of π-bonding, as shown for the fluoride complexes. 1. Introduction The early actinide elements Th-Am, with their 5f shell extending into the bonding region, may be considered the first true f elements in the Periodic Table. The 4f shell of the lanthanides is the first of its kind and therefore compact and corelike. The 5f shell contracts with nuclear charge for the later actinides and also approaches corelike character. Both lan- thanides and later actinides show thus a d-element-like “rare- earth” behavior. The most important consequences of 5f orbital involvement in early actinide chemistry are perhaps the existence of actinides in variety of oxidation states (e.g., from +III up to +VIII for Pu 1-3 ), a propensity to form bulky complexes with often unusual coordination arrangements unknown for transition metal systems, and a pronounced oxophilicity. Recently, a symmetry trend was noticed 2 in the molecular structures of isoelectronic 6d 0 5f 0 actinide oxide and oxyfluoride complexes. When going from lighter to heavier actinide centers along a given isoelectronic series, the molecular structure changes from less to more symmetric, with addition of an inversion center where stoichiometry allows. Experimentally known and previously discussed examples are the 6d 0 5f 0 AnO 2 q and AnO 4 q series (q denotes the overall charge of the complex needed to maintain a 6d 0 5f 0 configuration). ThO 2 is a bent C 2V system (R) 122.5°), 4 while UO 2 2+ is linear. 5 The computa- tionally characterized PaO 2 + and NpO 2 3+ are also linear. 2,6,7 In the AnO 4 q (An ) U, Np, Pu) series, UO 4 2- is tetrahedral, while NpO 4 - and PuO 4 are planar D 4h complexes. 2,7 Changes in structural preferences along isoelectronic series were observed computationally also for AnO 2 F q , AnO 2 F 2 q , AnF 8 q , 2 and AnO 3 q 8 complexes. This presumably general behavior was explained intuitively by increasing stabilization and bonding contributions of 5f orbitals with relatively unchanged 6d orbitals along the Th-Pu series, combined with different structural influences of f and d orbital bonding contributions. 2,9 Most theoretical investigations have concentrated on the 6d 0 5f 0 AnO 2 q series (An ) Th-U). 6,9-11 In ThO 2 , 6d orbital bonding contributions maximize covalent bonding in a bent structure, in analogy to the d 0 transition metal MO 2 q (M ) V, W, Mo) systems. 12,13 The f orbitals are not as strongly involved in bonding as to force a linear structure. Their overall role is not negligible, however, as can be seen from the O-Th-O angle of 122.5°, which is noticeably larger than the 102°-114° O-M-O angles in d 0 MO 2 q systems (M ) V, W, Mo). 9,10 In the heavier members of the AnO 2 q series, the f orbitals contract and stabilize (see above) and thus become appreciably bonding. The precise way in which this favors the linear uranyl and related structures is still a matter of argument. 6,9-11 5f Orbital ² Universita ¨t Wu ¨rzburg. ‡ Slovak Academy of Sciences. § Present address: Department of Chemistry, University of Helsinki, POB 55 (A. I. Virtasen aukio 1), FIN-00014 Helsinki, Finland. (1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; p 1468. (2) Straka, M.; Dyall, K. G.; Pyykko ¨, P. Theor. Chem. Acc. 2001, 106, 393. (3) Domanov, V. P.; Buklanov, G. V.; Lobanov, Yu. V. J. Nucl. Sci. Technol. 2002 (Suppl. 3), 579. (4) Gabelnick, S. D.; Reedy, G. T.; Chasanov, M. G. J. Chem. Phys. 1974, 60, 1167. (5) Gmelin Handbook of Inorganic Chemistry; Springer-Verlag: Berlin, 1983; Supplement A6. (6) Dyall, K. G. Mol. Phys. 1999, 96, 511 and references therein. (7) Bolvin, H.; Wahlgren, U.; Gropen, O.; Marsden, C. J. Phys. Chem. A 2001, 105, 10570 and references therein. (8) Hroba ´rik, P.; Straka, M.; Kaupp, M. Unpublished work. (9) Pyykko ¨, P.; Laakkonen, L. J.; Tatsumi, K. J. Am. Chem. Soc. 1989, 28, 1801 and references therein. (10) Tatsumi, K.; Hoffmann R. Inorg. Chem. 1980, 19, 2656 and references therein. (11) Wadt, W. R. J. Am. Chem. Soc. 1981, 103, 6053. (12) Pyykko ¨, P.; Tamm, T. J. Phys. Chem. A 1997, 101, 8107. (13) Kaupp, M. Chem.sEur. J. 1999, 5, 3631. Published on Web 02/02/2005 10.1021/ja044982+ CCC: $30.25 © 2005 American Chemical Society J. AM. CHEM. SOC. 2005, 127, 2591-2599 9 2591