New bimetallic Ni /Rh carbonyl clusters: synthesis and X-ray structure of the trigonal antiprismatic [NiRh 5 (CO) 14 ] 3 and body-centered cubic [NiRh 14 (CO) 28 ] 4 cluster anions Davide Collini, Cristina Femoni, Maria Carmela Iapalucci, Giuliano Longoni *, Per H. Svensson Dipartimento di Chimica Fisica ed Inorganica, Universita ` di Bologna, viale Risorgimento 4, 40136 Bologna, Italy Received 28 June 2002; accepted 10 October 2002 Dedicated to Dr. Pierre Braunstein in recognition of his outstanding contributions to chemistry. Abstract The reaction of the [Ni 6 (CO) 12 ] 2 dianion with [Rh(COD)Cl] 2 (COD /cyclooctadiene) in excess affords a complicate mixture of bimetallic Ni  /Rh clusters, from which the new [NiRh 5 (CO) 14 ] 3 and [NiRh 14 (CO) 28 ] 4 have been isolated and characterised. The [NiRh 14 (CO) 28 ] 4 tetraanionic cluster has been also obtained from reacting [Ni 2 Rh 12 (CO) 25 ] 4 and [NiRh 13 (CO) 25 ] 5 with [Rh(CO) 2 Cl] 2 , whereas [NiRh 5 (CO) 14 ] 3 has been more conveniently obtained by reduction with alkali metals or hydroxides of [NiRh 5 (CO) 15 ]  . All these new bimetallic Ni  /Rh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and were characterised by elemental analysis, spectroscopy and X ray diffraction studies. The structure of the diamagnetic [NiRh 5 (CO) 14 ] 3 trianion is identical to that of the homometallic [Co 6 (CO) 14 ] 4 tetraanion, the unique Ni atom being disordered over the 6 equiv. trigonal antiprismatic sites. The [NiRh 5 (CO) 14 ] 3 trianion is slowly and irreversibly degraded to a mixture of Ni(CO) 4 , [Rh(CO) 4 ]  and [Rh 7 (CO) 16 ] 3 upon exposure to an atmosphere of carbon monoxide. The [NiRh 14 (CO) 28 ] 4 tetranion displays a metal frame consisting in a hexacapped cube of rhodium atoms centered by the unique nickel atom. This metal frame has previously been found in the homometallic [Rh 15 (CO) 30 ] 3 cluster. However, at difference from the latter, [NiRh 14 (CO) 28 ] 4 features two less valence electrons and displays a shrinked cubic moiety. [NiRh 14 (CO) 28 ] 4 is slowly degraded by carbon monoxide and halide ions to give [NiRh 13 (CO) 25 ] 5 and other yet uncharacterised Ni  /Rh carbonyl clusters. All above Ni  /Rh clusters do not display protonation behaviour. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbonyl; Bimetallic cluster; Nickel; Rhodium; X ray structures 1. Introduction Several bimetallic Ni  /Rh carbonyl clusters have been isolated and structurally characterised, e.g. [NiRh 6 - (CO) 16 ] 2 , [1] [Ni 6 Rh 5 (CO) 21 ] 3 , [2,3] [Ni 10 Rh- (CO) 19 ] 3 , [3] [Ni 9 Rh 3 (CO) 22 ] 3 [3] [Ni 6 Rh 3 (CO) 17 ] 3 , [3] [NiRh 13 (CO) 25 ] 5 , [4] [Ni 2 Rh 12 (CO) 25 ] 4 , [4] and [Ni 5 Rh 9 (CO) 25 ] n (n /2, 3) [4]. The tetradecanuclear [NiRh 13 (CO) 25 ] 5 [Ni 2 Rh 12 (CO) 25 ] 4 and [Ni 5 Rh 9 - (CO) 25 ] n (n /2, 3), albeit isostructural, are not iso- electronic with the homometallic [Rh 14 (CO) 25 ] 4 , [5] and feature 182  /184 cluster valence electrons, rather than 180. Likely, this is a consequence of nickel, rather than rhodium centring of the pentacapped cubic metal frame, as well as progressive substitution of rhodium with nickel side-caps. Furthermore, nickel centring triggers redox aptitude in [NiRh 13 (CO) 25 ] 5 and [Ni 2 Rh 12 (CO) 25 ] 4 , and multivalence in [Ni 5 Rh 9 - (CO) 25 ] n (n /2, 3) [4]. It was, therefore, of interest to try to synthesise a pentadecanuclear bimetallic Ni  /Rh cluster to verify whether a mixed hcp  /bcc or a regular bcc geometry would result, to be respectively, compared with the * Corresponding author. Tel.:  /39-051-209 3711; fax:  /39-051-209 3690. E-mail address: longoni@ms.fci.unibo.it (G. Longoni). Inorganica Chimica Acta 350 (2003) 321  /328 www.elsevier.com/locate/ica 0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01545-1