2502 Organometallics zyxwvu 1988, zyxwvut 7, 2502-2506 and Cp*&e(amido) complexes (1 1,12), respectively. Re- action of 8 with LiCl in the presence of basic ligands; e.g., TMEDA gives Cp*2CeC1zLi(TMEDA)2 (3) and LiCH- (SiMe&, indicating that alkylation of Ce-C1 bonds by metathetical reactions with a main-group hydrocarbyl is thermodynamically very critical. Despite the paramagnetic character of these zyxwvutsr bis(pentamethylcyclopentadieny1)cerium complexes, 13C NMR spectra are readily interpreted and provide valuable information. Note Added in Proof. After submission of this paper Registry No. 1, 106043-06-5; 2,116635-57-5; 3, 116635-58-6; Evans et al. reported the synthesis of 4 and 5.46 4, 98789-69-6; 5, 111559-69-4; 6, 116635-59-7; 7, 113132-51-7; 8, 106333-13-5; 9,116635-60-0; 10, 106433-89-0; 11,116635-61-1; 12, 116635-62-2; CeC13, 7790-86-5; Ce02, 1306-38-3; Cp*Li, 51905-341; LiCH(SiMe3)2, 41823-71-6; t-BuCN, 630-18-2; (CP*~C~D)~, 116635-63-3; HN(SiMe3)2, 999-97-3; NaN(SiMe3)2, 1070-89-9; t-BuNH2, 75-64-9. Supplementary Material Available: zyx An ORTEP drawing and tables of atom coordinates, thermal parameters, bond distances and angles, and hydrogen coordinates for 8 (23 pages); a listing of structure factors for 8 (21 pages). Ordering information is given on any current masthead page. Acknowledgment* This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organi- zation for the Advancement of Pure Research (NWO). We thank Shell Research B.V. for financial support and P. Pasman, H. van der Heijden and E. J. M. de Boer for stimulating discussions. (46),Evans, W. J.; Olofson, J. M.; Zhang, H.; Atwood, J. L. zyx Organo- metallccs 1988, 7, 629. Photoexcitation of Tungsten Hexacarbonyl in Solutions Containing Pyridine-2-carbaldehyde Imine Ligands. Spectral Characterization and Reaction Kinetics of the Photoproduced Monodentate W zyxwv ( CO)5L Intermediate Denis P. Drolet, Lisa Chan, and Alistair J. Lees" Department of Chemistry, University Center at Binghamton, State University of New York, Binghamton, New York 13901 Received March 15, 1988 UV photolyses of W(CO)6 in solutions containing pyridine-2-carbaldehyde N-substituted imine (R-pyca) or 6-methyl-pyridine-2-carbaldehyde N-substituted imine (R-pyca') ligands produce transient species that are attributed to W(CO)& complexes, where the normally chelating ligands are coordinated in a monodentate configuration. Electronic absorption and Fourier transform infrared spectra recorded immediately following light excitation using rapid scanning techniques have enabled characterization of this monodentate W(CO)5L species. Kinetic data have been obtained for the chelation reactions of W(CO)5Lto form W(CO)4L and C O in each case the ring closure reaction proceeds via a first-order mechanism. From the ligand dependence of the kinetic data it is inferred that R-pyca scavenges the primary photoproduct exclusively at the pyridine nitrogen atom whereas R-pyca' initially coordinates at the aliphatic nitrogen atom. These kinetic results are discussed in terms of steric hindrance during the ligand scavenging process. Introduction Photochemical studies of metal carbonyl complexes have been greatly stimulated by the importance of these mol- ecules as models in mechanistic organometallic chemistry and in the design of homogeneous catalytic processes.l Although unsaturated species are believed to be involved in a number of these processes, direct spectral observation and quantitative measurements of their reactivity have rarely been reported. In fact, our current knowledge of the identity and reactivity of intermediates involved in ligand substitution events at low-valent transition-metal centers remains fairly limited. Recently, several reports have appeared concerned with the nature of reaction intermediates that are produced during the photochemical formation of chelated M(COI4L (1) (a) Henrici-Olive,G.; Olive, S. Coordination and Catalysis; Verlag Chemie: New York, 1977. (b) Geoffroy, G. L.; Wrighton, M. S. Or- ganometallic Photochemistry; Academic: New York, 1979. (c) Julliard, M.; Chanon, M. Chem. Rev. 1983,83,425. (d) Salomon, R. G. Tetrahe- dron 1983,39,485. (e) Moggi, L.; Juris, A.; Sandrini, D.; Manfrin, M. F. Reu. Chem. Intermed. 1984,5, 107. 0276-7333/88/2307-2502$01.50/0 complexes from M(CO)6 (M = Cr, Mo, W) solutions con- taining bidentate ligands (L).24 Spectral data obtained using rapid-scanning Fourier transform infrared spec- troscopy3 and diode-array UV-visible spectroscopy4 have provided a direct characterization of the monodentate M(C0)5Lintermediate, where L is a series of substituted 2,2'-bipyridine, 1,6diazabutadiene, or pyridine-2-carb- aldehyde imine ligands. Consequently, the photochemical formation of the chelated M(C0)4L complex is believed to proceed according to eq 1 and zyxw 2. In a further extension (1) M(C0)5L M(CO)4L + CO (2) M(CO)6 $ M(CO)5L + CO (2) Staal, L. H.; Stufkens, D. J.; Oskam, A. Inorg. Chim. Acta 1978, (3) Kazlauskas, R. J.; Wrighton, M. S. J. Am. Chem. SOC. 1982,104, 26, 255. 5784. (4) (a) Schadt, M. J.; Gresalfi, N. J.; Lees, A. J. zyx J. Chem. SOC., Chem. Commun. 1984, 506. (b) Schadt, M. J.; Gresalfi, N. J.; Lees, A. J. Inorg. Chem. 1985,24, 2942. (c) Schadt, M. J.; Lees, A. J. Inorg. Chem. 1986, 25,672. (d) Chan, L.; Lees, A. J. J. Chem. SOC., Dalton Trans. 1987,513. 0 1988 American Chemical Society