CO fluxionality in Rh 4 (CO) 12 and Rh 6 (CO) 16 q Brian T. Heaton a, * , Chacko Jacob a,1 , Ivan S. Podkorytov b , Sergey P. Tunik c a Department of Chemistry, University of Liverpool, Crown Street, P.O. Box 147, Liverpool L69 7ZD, UK b S.V. Lebedev Central Synthetic Rubber Research Institute, Gapalskaya 1, St. Petersburg 198035, Russia c Department of Chemistry, St. Petersburg University, Universitetskii pr., 26, St. Petersburg 198504, Russia Received 28 October 2005; received in revised form 1 December 2005; accepted 1 December 2005 Available online 3 July 2006 This paper is dedicated to Professor Mike Mingos, in celebration of his 65th birthday. Abstract Both Rh 4 (CO) 12 and Rh 6 (CO) 16 exhibit CO-fluxionality and modern, variable temperature, NMR methods allow the unambiguous assignment of the three terminal CO resonances and, for Rh 4 (CO) 12 , show that the mechanism of CO-fluxionality, which has been con- troversial for a long time, unambiguously involves the merry-go-round process; Rh 6 (CO) 16 , which was previously thought to be static, is also shown to be fluxional, although the rate of CO-exchange is much less than found for substituted derivatives, and possible pathways for this CO-exchange are discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: NMR; Rhodium carbonyl clusters; Mechanisms; CO fluxionality 1. Introduction Rh 4 (CO) 12 and Rh 6 (CO) 16 were amongst the earliest transition metal carbonyls to be discovered [1–5]. The X-ray structure of Rh 4 (CO) 12 has been determined on at least three occasions [6–8] and shown to contain three edge-bridging carbonyls, viz, Rh 4 (l 2 -CO) 3 (CO) 9 (see Fig. 1a); this structure consists of a Rh 4 -tetrahedron orien- tated within the icosahedral carbonyl framework to give an overall structure with C 3v symmetry. Early IR and Raman studies of Rh 4 (CO) 12 at room tem- perature [9,10], together with solid state 13 C NMR studies [11] and low temperature 13 C NMR studies in solution [12– 14], are all consistent with the structure shown in Fig. 1a. Thus, in agreement with this structure, the 13 C NMR spec- trum consists of three equally intense doublets due to the apical, equatorial and axial terminal CO’s and a triplet due to the edge-bridging CO’s. However, it has long been known from 1D, variable temperature 13 C NMR measure- ments that there is carbonyl scrambling in solution and, at +60 °C, there is a single 13 C resonance, which is a quintet as a result of coupling to four 103 Rh nuclei, which have become equivalent on the NMR time-scale at this temper- ature. This shows that the exchange is intra-molecular, but two mechanisms have been proposed [14–18] – the first proposal, known as the merry-go-round process, was made by Cotton in 1966 [15] and involves opening and closing of the l 2 -CO’s so as to move them around the cluster in concerted steps. The other proposed mechanism envizages concerted movements of the metal skeleton within carbonyl polyhedra [17,18]. Recent variable pressure 13 C NMR 0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.12.002 q This paper celebrates the 65th birthday of Mike Mingos and the enormous contributions he has made to Inorganic Chemistry. Mike and I (BTH) were research students together in Professor Chatt’s laboratory at the University of Sussex and both became interested in cluster chemistry as a result of Paolo Chini, the father of cluster chemistry, spending six months working as a Visiting NATO Fellow in the same laboratory (1965). * Corresponding author. Tel.: +44 151 794 3524; fax: +44 151 794 3540. E-mail address: bth@liv.ac.uk (B.T. Heaton). 1 Present address: Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833. www.elsevier.com/locate/ica Inorganica Chimica Acta 359 (2006) 3557–3564