Bonding in Elongated Dihydrogen Complexes. Theoretical Analysis of the Electron Density in [ML n (H‚‚‚H)] Species Feliu Maseras,* ,†,‡ Agustı ´ Lledo ´s,* ,† Miquel Costas, § and Josep M. Poblet § Unitat de Quı ´mica Fı ´sica, Departament de Quı ´mica, Universitat Auto ` noma de Barcelona, 08193 Bellaterra, Barcelona, Catalonia, Spain, and Departament de Quı ´mica, Universitat Rovira i Virgili, 43005 Tarragona, Catalonia, Spain Received December 5, 1995 X The geometry of a series of four [ML n “H 2 ”] complexes (W(H 2 )(CO) 3 (PR 3 ) 2 , IrH(H‚‚‚H)- Cl 2 (PR 3 ) 2 , [Os(H‚‚‚H)(NH 2 (CH 2 ) 2 NH 2 ) 2 (RCO 2 )] + , and OsH 4 (PR 3 ) 3 ) spanning a large range of H-H values is optimized at the B3LYP computational level, yielding satisfactory agreement with available neutron-diffraction data. The electron density resulting from these theoretical calculations is analyzed afterward within the “atoms in molecules” formalism, resulting in a positive assignment of the complexes W(H 2 )(CO) 3 (PR 3 ) 2 and IrH(H‚‚‚H)Cl 2 (PR 3 ) 2 as dihydrogen complexes and of the complexes [Os(H‚‚‚H)(NH 2 (CH 2 ) 2 NH 2 ) 2 (RCO 2 )] + and OsH 4 - (PR 3 ) 3 as dihydride complexes. Introduction Characterization of transition metal dihydrogen com- plexes has probably been one of the most exciting developments in coordination chemistry in the last decade. 1 Their discovery has reshaped the view on the way σ bonds interact with metal atoms. Only 15 years ago, it was considered that coordination of a hydrogen molecule to a metal complex leads necessarily to the breaking of the H-H bond and the formation of two M-H bonds, resulting in a dihydride complex with the bonding structure shown in the left-hand side of Chart 1. Nowadays, the existence of species where the hy- drogen bond is not broken, namely dihydrogen com- plexes, is fully accepted. Dihydrogen complexes may in principle be represented by any of the bonding struc- tures in the right-hand side of Chart 1. Bonding in these species is already well understood. 2 There are an ever-growing number of reports concerning them in the literature, and the effect of their presence in a number of different processes is starting to be appreciated. Moreover, all developments in their chemistry have a direct impact on the even wider field of σ bond activa- tion. 3 Nevertheless, there remain dark areas in the char- acterization itself of dihydrogen complexes. The draw- ings in Chart 1 imply that the reaction ML n + H 2 f ML n “H 2 ” can give rise to either a dihydride complex or to a dihydrogen complex but that there is no other possible product in between. That is, all complexes with this stoichiometry should belong to one of the two classes. There is indeed a range of H-H distances considered “normal” for dihydrogen complexes (shorter than 0.85 Å) and another range considered “normal” for dihydride complexes (larger than 1.50 Å), and neutron diffraction experiments 4-6 have allowed the unequivocal assignment of a number of species 4-6 according to this criterion. However, even within the still scarce number of neutron diffraction studies available on this type of species, 4-9 there are some examples which lie outside the “common sense” ranges just mentioned. 7-9 These are the so-called elongated dihydrogen complexes, which † Universitat Auto `noma de Barcelona. ‡ Current address: Laboratoire de Structure et Dynamique des Syste `mes Mole ´culaires et Solides, UMR 5636, Universite ´ de Montpel- lier II, 34095 Montpellier Cedex 5, France. § Universitat Rovira i Virgili. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) (a) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120-128. (b) Crabtree, R. H. Acc. Chem. Res. 1990, 23, 95-101. (c) Jessop, P. J.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155-284. (d) Heinekey, D. M.; Oldham, W. J., Jr. Chem. Rev. 1993, 93, 913-926. (2) (a) Burdett, J. K.; Eisenstein, O.; Jackson, S. A. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: Weinheim, Germany, 1991; pp 149-184. (b) Lin, Z.; Hall, M. B. Coord. Chem. Rev. 1994, 135, 845-879. (3) (a) Crabtree, R. H.; Hamilton, D. G. Adv. Organomet. Chem. 1988, 28, 299-338. (b) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789-805. (4) (a) Ricci, J. S.; Koetzle, T. F.; Bautista, M. T.; Hofstede, T. M.; Morris, R. H.; Sawyer, J. F. J. Am. Chem. Soc. 1989, 111, 8823-8827. (b) Van Der Sluys, L. S.; Eckert, J.; Eisenstein, O.; Hall, J. H.; Huffman, J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 4831-4841. (c) Kubas, G. J.; Burns, C. J.; Eckert, J.; Johnson, W. W.; Larson, A. C.; Vergamini, P. J.; Unkefer, C. J.; Khalsa, G. R. K.; Jackson, S. A.; Eisenstein, O. J. Am. Chem. Soc. 1993, 115, 569-581. (5) (a) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. Soc. 1984, 100, 451-452. (b) Vergamini, P. J.; Wasserman, H. J.; Koetzle, T. F.; Kubas, G. J. Unpublished work cited in footnote 9 of: Kubas, G. J.; Unkefer, C. J.; Swanson, B. J.; Fukushima, E. J. Am. Chem. Soc. 1986, 108, 7000- 7009. (6) Hart, D. W.; Bau, R.; Koetzle, T. F. J. Am. Chem. Soc. 1977, 99, 7557-7564. (7) (a) Bramner, L.; Howard, J. A. K.; Johnson, O.; Koetzle, T. F.; Spencer, J. L.; Stringer, A. M. J. Chem. Soc., Chem. Commun. 1991, 241-243. (b) Klooster, W. T.; Koetzle, T.; Jia, G.; Fong, T. P.; Morris, R. H.; Albinati, A. J. Am. Chem. Soc. 1994, 116, 7677-7681. (8) Albinati, A.; Bakhmutov, V. I.; Caulton, K. G.; Clot, E.; Eckert, J.; Eisenstein, O.; Gusev, D. G.; Grushin, V. V.; Hauger, B. E.; Klooster, W. T.; Koetzle, T. F.; McMullan, R. K.; O’Loughlin, T. J.; Pe ´lissier, M.; Ricci, J. S.; Sigalas, M. P.; Vymenits, A. B. J. Am. Chem. Soc. 1993, 115, 7300-7312. (9) Hasegawa, T.; Li, Z.; Parkin, S.; Hope, H.; McMullan, R. K.; Koetzle, T. F.; Taube, H. J. Am. Chem. Soc. 1994, 116, 4352-4356. Chart 1 2947 Organometallics 1996, 15, 2947-2953 S0276-7333(95)00936-8 CCC: $12.00 © 1996 American Chemical Society + +