Assignment of the Electronic Spectra of [Mo(CN) 8 ] 4- and [W(CN) 8 ] 4- by Ab Initio Calculations M. F. A. Hendrickx,* ,† V. S. Mironov, ‡ L. F. Chibotaru, † and A. Ceulemans † Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium, and Institute of Crystallography, Russian Academy of Sciences, Leninskii Prosp. 59, 117333 Moscow, Russia Received November 6, 2003 CASPT2 calculations are performed on the dodecahedral and square antiprismatic isomers of the [Mo(CN) 8 ] 4- and [W(CN) 8 ] 4- complexes. The high-energy experimental bands above 40000 cm -1 are assigned to MLCT transitions. The experimental observed trend of the extinction coefficients for the molybdenum and tungsten complex is reproduced by our CASSCF oscillator strengths. All bands below 40000 cm -1 can be ascribed to ligand-field transitions, although small contributions from forbidden MLCT transitions cannot be excluded. In order to account for all experimental bands in the electronic spectrum of these octacyanocomplexes, a dynamic equilibrium in solution between the two isomeric forms must be hypothesized. Spin-orbit coupling effects are found to be more important for the square antiprismatic isomers; in particular, large singlet-triplet mixings are calculated for this isomer of [W(CN) 8 ] 4- . Ligand- field and Racah parameters as well as spin-orbit coupling constants are determined on the basis of the calculated transition energies. The obtained values for these parameters support the recently proposed model for exchange interactions in magnetic clusters and networks containing pentavalent octocyanometalates of molybdenum and tungsten. Introduction During the past years interest in the cyanocomplexes was revived due to the fact that they are building blocks of magnetic clusters and networks. 1-11 They are of potential importance for the development of single molecule magnets. It has been shown that the incorporation in cyano-bridged compounds of second- and third-row transition metal cations with diffuse d valence orbitals leads 12-14 to strong magnetic exchange couplings between the metal centers. A thorough understanding of the origin of the ferromagnetic coupling is currently the subject of theoretical research. Beside quantum chemical investigations, 15-17 models for kinetic exchange interaction based on Anderson’s theory of superexchange 18 have also been proposed. 19-21 The efficiency of the model * To whom correspondence should be addressed. E-mail: marc.hendrickx@chem.kuleuven.ac.be. ² University of Leuven. ‡ Russian Academy of Sciences. (1) Mallah, T.; Auberger, C.; Verdaguer, M.; Veillet, P. J. Chem. Soc., Chem. Commun. 1995, 61. (2) El Fallah, S. M.; Rentschler, E.; Caneschi, A.; Sessoli, R.; Gatteschi, D. Angew. Chem. 1996, 108, 2081-2083; Angew. Chem., Int. Ed. Engl. 1996, 35, 1947. (3) Scuiller, A.; Mallah, T.; Verdaguer, M.; Nivorozhkin, A.; Tholence, J.-L.; Veillet, P. New J. Chem. 1996, 20, 1. (4) Heinrich, J. L.; Berseth, P A.; Long, J. R. J. Chem. Soc., Chem. Commun. 1998, 1231. (5) Ferlay, S.; Mallah, T.; Quahe `s, R.; Velliet, P.; Verdaguer, M. Inorg. Chem. 1999, 38, 229. (6) Mallah, T.; Marvilliers, A.; Rivie `re, E. Phil. Trans. R. Soc. London, Ser. A 1999, 357, 3139. (7) Griebler, W.-D.; Babel, D. Z. Naturforsch., B: Chem. Sci. 1982, 37, 832. (8) Gadet, V.; Mallah, T.; Castro, I.; Verdaguer, M.; Veillet, P. J. Am. Chem. Soc. 1992, 114, 9213. (9) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554-1557. (10) Ferlay, S.; Mallah, T.; Quahe `s, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (11) Holmes, S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593. (12) Shores, M. P.; Sokol, J. J.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 2279. (13) Sokol, J. J.; Hee, A. G.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 7656. (14) Bennett, M. V.; Long, J. R. J. Am. Chem. Soc. 2003, 125, 2394. (15) Eyert, V.; Siberchicat, B.; Verdaguer, M. Phys. ReV.B 1997, 56, 8959. (16) Harrison, N. M.; Searle, B. G.; Seddon, E. A. Chem. Phys. Lett. 1997, 266, 507. (17) Nishino, M.; Yoshioko, Y.; Yamaguchi, K. Chem. Phys. Lett. 1998, 297, 51. (18) Anderson, P. W. Phys. ReV.B 1959, 115, 2. (19) Weihe, H.; Gu ¨del, H. U. Comments Inorg. Chem. 2000, 22, 75. (20) Chibotaru, L. F.; Mironov, V. S.; Ceulemans, A. Angew. Chem., Int. Ed. 2001, 40, 4429. (21) Mironov, V. S.; Chibotaru, L. F.; Ceulemans, A. J. Am. Chem. Soc. 2003, 125, 9750. Inorg. Chem. 2004, 43, 3142-3150 3142 Inorganic Chemistry, Vol. 43, No. 10, 2004 10.1021/ic035282t CCC: $27.50 © 2004 American Chemical Society Published on Web 04/14/2004