RHODATHIABORANES WITH ANOMALOUS ELECTRON COUNTS - SYNTHESIS, STRUCTURE AND REACTIVITY

Analysis of the structures of 8,8-(PPh3)(2)-8,7-nido-RhSB9H10 and 9,9- (PPh3)(2)-9,7,8-nido-RhC2B8H11 by RMS misfit calculations has confirme d that these rhodaheteroboranes possess nido 11-vertex cluster geometr ies in apparent contravention of Wade's rules. However, examination of the molecular structures of both species shows that the {RhP2} planes are inclined by ca. 66 degrees with respect to the metal-bonded SB3 o r CB3 faces, and that two weak ortho-CH ... Rh agostic interactions oc cupy the vacant co-ordination position thereby created. As a consequen ce of these agostic bonds the Rh atom, and hence the overall cluster, is provided with an additional electron pair, meaning that their nido structures are now fully consistent with Wade's rules. The chelated di phosphine compound 8,8-(dppe)-8,7-nido-RhSB9H10 is similar to the PPh3 compound in showing the same agostic bonding. Attempts to prepare a b is-P(OMe)(3) analogue result in ligand scavenging and the formation of 8,8,8-{P(OMe)(3)}(3)-8,7-nido-RhSB9H10. Similarly, reaction between C s[6-arachno-SB9H12] and RhCl(dmpe)CO does not result in CO loss but in formation of 8,8-(dmpe)-8-(CO)-8,7-nido-RhSB9H10, shown to exist as a mixture of two of three possible rotamers. Deprotonation of 8,8-(PPh3 )(2)-8,7-nido-RhSB9H10 and 8,8-(dppe)-8,7-nido-RhSB9H10 with MeLi yiel ds the anions [1,1-(PPh3)(2)-1,2-closo-RhSB9H9](-) and [1,1-dppe-1,2-c loso-RhSB9H9](-), respectively, with octadecahedral cage structures. I t is argued that anion formation causes the agostic bonding to be 'swi tched-off' and results in the cluster adopting the close architecture predicted by Wade's rules. This structural change is fully reversible on reprotonation, and if reprotonation of [1,1-(dppe)-1,2-closo-RhSB9H 9](-) is carried out in MeCN, the product 8,8-(dppe)-8-(MeCN)-8,7-nido -RhSB9H10 forms. Interestingly, 8,8-(dppe)-8-(MeCN)-8,7-nido-RhSB9H10 reconverts to 8,8-(dppe)-8,7-nido-RhSB9H10 on standing in CDCl3, sugge sting that the agostic bonding is sufficiently strong to displace co-o rdinated MeCN. All new compounds are fully characterised by multinucle ar NMR spectroscopy and, in many cases, by single crystal X-ray diffra ction. (C) 1998 Elsevier Science S.A.