Electronic Structure of Bis(2,4-pentanedionato-O,O′)oxovanadium(IV). A Photoelectron Spectroscopy, Electronic Spectroscopy, and ab Initio Molecular Orbital Study Santo Di Bella, Giuseppe Lanza, Antonino Gulino, and Ignazio Fragala ` * Dipartimento di Scienze Chimiche, Universita ` di Catania, V.le A. Doria 8, 95125 Catania, Italy ReceiVed NoVember 10, 1995 X The electronic structure of the title VO(acac) 2 complex has been investigated using effective core potential configuration interaction ab initio calculations, UV-photoelectron spectroscopy, and electronic spectroscopy. The metal-ligand bonding with the equatorial acac - ligands is dominated by σ interactions involving the filled ligand orbitals and the empty orbitals of the d 1 vanadium(IV) ion. The oxovanadium interactions involve a larger metal-d participation thus resulting in a strong V-O bonding having partial triple-bond character. Additional three-orbital-four-electron stabilizing interactions involving the filled acac - MOs and the oxovanadium orbitals further reinforce both the axial and equatorial bonds. The unpaired metal-d electron is completely localized in the nonbonding d x 2 -y 2 orbital. The low ionization energy of the photoelectron spectrum has been fully assigned on the basis of combined ∆SCF and configuration interaction calculations. The same theoretical approach has, in addition, provided a good fitting of frequencies associated with “d-d” and charge transfer electronic transitions. Introduction The oxovanadium(IV) ion (VO 2+ ) plays a dominant role in the vanadium(IV) chemistry. 1 Among its complexes, the vanadyl acetylacetonate (VO(acac) 2 ) is certainly the most representative species. VO(acac) 2 has been the subject of many experimental investigations devoted to its interesting paramagnetic 2-4 and physicochemical 1,5,6 properties, mainly associated with the 3d 1 electronic configuration. VO(acac) 2 possesses a square-pyramidal arrangement of the five coordinated oxygen atoms, with the vanadium atom close to the center of gravity of the pyramid. Early X-ray crystal structure determinations 7 and a more recent gas-phase electron diffraction study 8 have pointed to a monomeric structure of C 2V symmetry, with the VdO bond along the 2-fold axis. Magnetic susceptibility and ESR data have indicated a magnetic moment and paramagnetic resonance factors both consistent with a single unpaired electron. 1-3 In spite of these experimental efforts, less attention has been paid to theoretical aspects associated with the VO(acac) 2 molecule, since only ligand-field 9 and semiem- pirical calculations 10,11 have appeared in the literature. In this context, we embarked on a combined study involving ab initio calculations, UV-photoelectron (PE) spectroscopy, 12 and electronic spectroscopy to investigate the electronic structure of VO(acac) 2 . The very good resolution of its PE spectra offers, in addition, the possibility to probe theoretical results by a direct one-to-one comparison of experimental ionization energies (IEs) with calculated data. Experimental Section VO(acac)2 (Aldrich) was purified by sublimation in vacuo. He I and He II PE spectra were measured as described elsewhere. 12 Resolution measured on the He 1s -1 line was around 25 meV. The PE spectra were recorded in the 160-180 °C temperature range. The spectra were deconvoluted by fitting the spectral profiles with a series of asymmetrical Gaussian curves after subtraction of the background. 13 The area bands thus evaluated can be affected by errors smaller than 5%. Optical absorption UV-vis spectra were recorded with a Beckman DU 650 spectrophotometer. Theoretical Methods Ab initio effective core potential (ECP) were employed in the molecular calculations using the restricted Hartree-Fock (RHF) method for the closed-shell states, and the restricted open Hartree-Fock (ROHF) method for the open-shell states. The ionization energies of the lower lying ionic states of each symmetry were evaluated using ∆SCF procedures, which account only for relaxation contribution to the total reorganization energy. The effect of electron correlation was considered by using the configuration interaction (CI) procedure including all-single and double-excitations (CISD) from the single HF reference. Since a complete CISD treatment would generate an enormous number of configuration state functions (CSFs), the treatment was limited to using the highest 10 occupied MOs and the lowest 10 virtual orbitals having dominant metal 3d, 4s, 4p and ligand π4 character (10/10 expansion, ∼40 000 CSFs generated). In addition, the ionization energies of the two most stable states of the cation ( 3 B2 and 1 A1) were also calculated by a more extended CISD 18/10 expansion (∼430 000 CSFs generated). Because of the variational collapse, the ∆SCF procedure allows the calculation of the ionization energy only for the lowest-lying state of each symmetry. Therefore, for those states only CI calculations with the appropriate SCF orbitals were performed. The IEs of the higher-lying ionic states were evaluated using, as reference, the SCF orbitals of the lowest state of each symmetry, by solving the X Abstract published in AdVance ACS Abstracts, May 1, 1996. (1) Selbin, J. Chem. ReV. 1965, 65, 153. (2) Chasteen, D. N. In Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum: New York, 1981; p 53. (3) (a) Yordanov, N. D.; Zdravkova, M. Polyhedron 1993, 12, 635. (b) Stemp, E. D. A.; Eaton, G. R.; Eaton, S. S.; Maltempo, M. M. J. Chem. Soc., Chem. Commun. 1988, 61. (c) Baker, G. J.; Rayno ´r, B. J. J. Chem. Soc., Faraday Trans. 1 1988, 84, 4267. (4) Gregson, A. K.; Mitra, S. Inorg. Chim. Acta 1980, 45, L121. (5) (a) Valek, M. H.; Yeranos, W. A.; Basu, G.; Hon, P. K.; Belford, R. L. J. Mol. Spectrosc. 1971, 37, 228. (b) Selbin, J.; Ortolano, T. R. J. Inorg. Nucl. Chem. 1964, 26, 37. (6) Tantrawong, S.; Styring, P.; Goodby, J. W. J. Mater. Chem. 1993, 3, 1209. (7) (a) Dodge, R. P.; Templeton, D. H.; Zalkin, A. J. Chem. Phys. 1961, 35, 55. (b) Hon, P.-K.; Belford, L. R.; Pfluger, C. E. J. Chem. Phys. 1965, 43, 3111. (8) Forsyth, G. A.; Rice, D. A.; Hagen, K. Polyhedron 1990, 9, 1603. (9) Amoro ´ s, P.; Iba ´n ˜ ez, R.; Beltra ´n, A.; Beltra ´n, D. J. Chem. Soc., Dalton Trans. 1988, 1665. (10) Ballhausen, C. J.; Gray, H. B. Inorg. Chem. 1962, 1, 111. (11) Ritschl, F.; Haberlandt, H. J. Mol. Struct. (THEOCHEM.) 1988, 180, 45. (12) Di Bella, S.; Fragala `, I.; Granozzi, G. Inorg. Chem. 1986, 25, 3997. (13) Casarin, M.; Ciliberto, E.; Gulino, A.; Fragala `, I. Organometallics 1989, 8, 900. 3885 Inorg. Chem. 1996, 35, 3885-3890 S0020-1669(95)01457-1 CCC: $12.00 © 1996 American Chemical Society