1008 We thank the Greek Secretariat of Research and Technology for financial support. TWO ISOMERS OF C18H19C1N402 Supplementary data for this paper are available from the IUCr electronic archives (Reference: GS 1035). Services for accessing these data are described at the back of the journal. References Papaioannou, D. (1998). Unpublished results. Papaioannou, D., Stavropoulos, G., Sivas, M., Barlos, K., Francis, G. W., Aksnes, D. W. & Maartmann-Moe, K. (1991). Acta Chem. Scand. 45, 99-104. Sheldrick, G. M. (1990). Acta Cr)'st. A46, 467--473. Sheldrick, G. M. (1997). SHELXL97. Program for the Refinement of Crystal Structures. University of G6ttingen, Germany. Spek, A. L. (1990). Acta Crvst. A46, C-34. Stoe & Cie (1988a). DIF4. Diffractometer Control Program. Version 6.2. Stoe & Cie, Darmstadt, Germany. Stoe & Cie (1988b). REDU4. Data Reduction Program. Version 6.2. Stoe & Cie, Darmstadt, Germany. information on the dependence of the C--C distance as a function of the C substituent, the crystal structure of (PMePh3)(7,8-Et2-7,8-C2BgH10), (I), is now reported. El C/ _Et ..... - /:.. MePh3P+. (I) An anionic structural unit of the salt is shown in Fig. 1. The methyl group of one ethyl arm bonded to the cage C atom is orientated up from the upper belt (atoms C7, C8, B9, B 10 and B 11) of the boron cage. The methyl group of the other ethyl group is approximately in the same plane of the upper-belt atoms. The bond angles and distances in the anion and cation are normal, but some distortions are found in the anion. Acta Cryst. (1999). C55, 1008-1009 (PMePh3)(7,8-Et2-7,8-nido-C2B9H10) REIJO SILLANP,X,A, a JOSEFINA PEDRAJAS, b CLARA VII~AS, b FRANCES TEIXIDOR b AND RAIKKO KIVEKAS c "Department of Chemistry, University of Turku, FIN-20014 Turku, Finland, blnstitut de Ci~ncia de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain, and "Department of Chemistry, University of Helsinki, FIN- 00014 Helsinki, Finland. E-maih sillanpa@ utu.fi (Received 2 June 1998. accepted 8 Februar)' 1999) Abstract Methyltriphenylphosphonium 7,8-diethyl-7,8 -dicarba- nido-undecaborate(1-), C19HIsP+.C6H?0B9-, is a salt which is formed from the cationic phosphonium ion and anionic nido carborane cage. The C---C distance in the boron cage is 1.567 (4) A. Comment Our studies concerning closo-1,2-dicarbadodeca- borane(12)s have led us to conclude that the C--C distance in the cage can be modified (Sillanp~i~i et al., 1996). In the case of relevant nido cages, the elongation seems to be less striking. For example, in nido-(7,8- C2B9Hl2)- the C--C distance is 1.542 (3).~ (Buchanan et al., 1990) and in nido-(7,8-Ph2-7,8-C2B9Hio)- (NHEt~ salt) 1.590 (5).4, and for [NMe3(CH2Ph)] + salt 1.602 (3)A (Cowie et al., 1993). In order to get more © 1999 Intemational Union of Crystallography Printed in Great Britain - all rights reserved C1 ?Cq 0 C7 ? "~ d B2 ' ~ O Fig. 1. ORTEPII (Johnson. 1976) plot of the anionic part of (1) in the asymmetric unit. Only the heavy atoms have been labelled. Displacement ellipsoids are drawn at the 20ok probability level. In the lower belt (B2, B3, B4, B5 and B6), the B--B distances are in the range 1.730 (7)-1.806 (7) ,4, (Table 1), while in the upper belt the C7---C8 bond length is 1.567 (4)(the shortest° bond) and the B9-- B10 bond length is 1.843(7)A (the longest bond). Bond angles in the lower belt range from 106.0(3) to 109.8 (3) ° and in the upper belt from 101.5 (3) to 114.5 (3) ° . Alkyl substituents on C atoms in 7,8-dicarbaboranes seem to have quite a small effect on the C--C bond distance. Aromatic substituents, compared with H sub- stituents, lengthen the C--C distance by about 0.05 ~,. Generally this follows with that which has been found for relevant closo cages, but in the nido case this effect is much weaker. Acta Cr},stallographica Section C ISSN 0108-2701 © 1999