Synthesis and Molecular and Electronic Structure of an Unusual Paramagnetic Borohydride Complex Mo(NAr) 2 (PMe 3 ) 2 (η 2 -BH 4 ) ² Andrey Y. Khalimon, ‡ Jason P. Holland, § Radoslaw M. Kowalczyk, | Eric J. L. McInnes, | Jennifer C. Green,* ,§ Philip Mountford,* ,§ and Georgii I. Nikonov* ,‡ Chemistry Department, Brock UniVersity, Glenridge AVe. 500, St Catharines, ON L2S3A1, Canada, Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, U.K., and EPSRC c. w. EPR SerVice Centre, School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K. Received September 15, 2007 Reaction of Mo(NAr) 2 Cl 2 (DME) (Ar ) 2,6-C 6 H 3 i Pr 2 , DME ) 1,2-dimethoxyethane) with NaBH 4 and PMe 3 in THF formed the paramagnetic Mo(V) d 1 borohydride complex Mo(NAr) 2 (PMe 3 ) 2 (η 2 -BH 4 )(1). Compound 1, which was characterized by EPR spectroscopy and X-ray diffraction analysis, provides a rare example both of a paramagnetic bis(imido) group 6 compound and a structurally characterized molybdenum borohydride complex. Density functional theory calculations were used to determine the electronic structure and bonding parameters of 1 and showed that it is best viewed as a 19 valence electron compound (having a primarily metal-based SOMO) in which the BH 4 - ligand behaves as a σ-only, 2-electron donor. Introduction Transition metal hydrides are ubiquitous in coordination and organometallic chemistry and usually conform to the effective electron number rule, i.e., exhibit either an 18 or 16 (for d 8 complexes) valence electron count. 1 Although complexes with an odd valence shell have become quite common, odd-electron hydrides (e.g., the 17 and 19 valence electron species Cp 2 Ti(η 2 -BH 4 ) and Ni(Triphos)(η 2 -BH 4 ), respectively) are relatively rare. 2-4 Steric protection can stabilize electron-deficient centers, as is the case in Poli’s 15 valence electron complex (η-C 5 H 2 Bu t 3 )Mo(H) (PMe 3 ) 2 , 5 but “electron rich” 19e and 20e configurations usually only occur if there are strong π-accepting ligands, such as CO, NO, and PF 3 , which can delocalize the “excess” electron density from the metal. 1 This research originally stemmed from our interest in studying hydride derivatives in metallocene-like ligand platforms. 6 N-based ligands isolobal with Cp - , such as R 3 PN - , RN 2- , and cyclic triamines, have been recently successfully applied for systematic design of post-metal- locene ligand environments, primarily for the application in catalytic olefin polymerization, 7 but relatively little is known about their hydride derivatives. 8 Because the trihydrides [Cp 2 - * To whom correspondence should be addressed. E-mail: philip.mountford@chemistry.oxford.ac.uk (P.M.), gnikonov@brocku.ca (G.I.N.). ² Dedicated to Professor Martyn Poliakoff on the occasion of his 60th birthday. ‡ Brock University. § University of Oxford. | The University of Manchester. (1) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; Wiley: New York, 2005. (b) Crabtree, R. H. Hydrogen and Hydrides as Ligands. In ComprehensiVe Inorganic Chemistry; Wilkinson, G., Ed.; Pergamon: London, Chapter 19. (2) (a) Poli, R. In Recent AdVances in Hydride Chemistry; Poli, R., Peruzzini, M., Eds.; Elsevier: Amsterdam, 2001; Chapter 6. (b) Pleune, B.; Morales, D.; Meunier-Prest, R.; Richard, P.; Collange, E.; Fettinger, J. C.; Poli, R. J. Am. Chem. Soc. 1999, 121, 2209. (c) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 2000, 19, 3931. (d) Jewson, J. D.; Liable-Sands, L. M.; Yap, G. P. A.; Rheingold, A. L.; Theopold, K. H. Organometallics 1999, 18, 300. (e) Kersten, J. L.; Rheingold, A. 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