Intramolecular hydrogen bonding in transition metal clusters: NMR evidence for an Os  /HÁÁÁH /S interaction in H(m-H)Os 3 (CO) 10 (HSR) (R /ethyl, cyclopentyl) Silvio Aime, Michele R. Chierotti, Roberto Gobetto *, Andrea Russo, Marc J. Stchedroff Dipartimento di Chimica Inorganica, Chimica Fisica, Chimica dei Materiali, Universita ` degli Studi di Torino, via Pietro Giuria 7, 10125 Turin, Italy Received 3 February 2003; accepted 22 February 2003 Abstract The reaction of H 2 Os 3 (CO) 10 with RSH (R /CH 2 CH 3 , cyclopentylthiol) at low temperatures in CD 2 Cl 2 affords three H(m- H)Os 3 (CO) 10 (RSH) adducts, where, by the 13 C NMR carbonyl spectrum, it can be shown that the thiol ligand occupies an axial position. The presence of an intermolecular MHÁÁÁHS interaction in the main isomer has been determined by longitudinal relaxation time measurements. The isolation of the H,H dipolar contribution to the terminal hydride relaxation affords a value of 1.97 A ˚ for the ‘unconventional’ hydrogen /hydrogen interaction in H(m-H)Os 3 (CO) 10 (RSH) (R /cyclopentylthiol). An intramolecular proton transfer from the thiol ligand RSH to the terminal hydride, via a dihydrogen bonded species, ultimately losing an H 2 molecule. leading to the formation of the stable product (m-H)Os 3 (CO) 10 (m-SR) is proposed. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Unconventional hydrogen bond; Cluster; Osmium; Thiol; NMR; T 1 1. Introduction The interaction of the M  /HÁÁÁH  /X type where X is an electronegative main group atom such as C, N, O has been investigated extensively by crystallographic and spectroscopic analysis [1]. It has been shown that such interactions, called dihydrogen bonds, may significantly contribute to the stabilisation of organometallic struc- tures in mononuclear complexes and may play an important role in a large number of the proton transfer reactions catalysed by transition metal hydrides [2]. In particular it has been suggested that such interac- tions represent an intermediate step in intramolecular proton transfers (within a transition metal complex) from the ligand, acting as an acid, to the hydride leading to a h 2 -H 2 system. When reversed (transfering from the coordinated dihydrogen to a co-ligand) the co-ligand then acts as a base according to Eq. (1) [3]: M(H)(HL?)(L) n l M(h-H 2 )(L) n L? (1) So far, little is known about the factors which control this intramolecular proton transfer. We have found that the dihydrogen bond interaction is also present in transition metal clusters [4  /6]: for example, an intramolecular hydrogen bond is present in H(m-H)Os 3 (CO) 10 L derivatives where L is a terminally bound amine or imine ligands [4]. In principle, deriva- tives of formula H(m-H)Os 3 (CO) 10 L containing the N- donor ligand in an axial position should be present in two isomeric forms, that is with the ligand either cis or trans to the terminal hydride (Scheme 1). However, the observation of isomer I a as the sole product has been ascribed to the stabilisation effect arising from the formation of an intramolecular hydro- gen bonding interaction between the N /H moiety and the terminal hydride ligand. NMR evidence for this interaction has been obtained by the measurement of the proton T 1 relaxation times of the hydride resonances and from this the r H,H distance between H T and the * Corresponding author. Tel.:  /39-011-6707-523; fax:  /39-011- 6707-855. E-mail address: roberto.gobetto@unito.it (R. Gobetto). Inorganica Chimica Acta 351 (2003) 251  /255 www.elsevier.com/locate/ica 0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(03)00120-8