Di-l-benzoato-bis[dicarbonyl(pyri- dine)ruthenium(I)] (new polymorph) and di-l-trifluoroacetato-bis[di- carbonyl(pyridine)ruthenium(I)] Glen B. Deacon, a Pauline Pearson, a Brian W. Skelton, b * Leone Spiccia a and Allan H. White b a Department of Chemistry, Monash University, Victoria 3800, Australia, and b Chemistry, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Correspondence e-mail: bws@crystal.uwa.edu.au Received 8 August 2003 Accepted 17 October 2003 Online 14 November 2003 The syntheses and crystal structure determinations of a pair of `sawhorse' dimers are reported, viz. [Ru 2 (C 6 H 5 CO 2 ) 2 - (C 5 H 5 N) 2 (CO) 4 ] [a new polymorph, cf. Kepert, Deacon, Spiccia, Fallon, Skelton & White (2000). J. Chem. Soc. Dalton Trans. pp. 2867±2874] and [Ru 2 (CF 3 CO 2 ) 2 (C 5 H 5 N) 2 (CO) 4 ]. The RuRu distances are 2.6724 (2) and 2.7122 (5) A Ê , respectively. Comment An earlier report (Kepert et al., 2000) records the syntheses and chemical and structural characterizations of a number of binuclear ruthenium(I) `sawhorse' complexes, viz. [Ru(RCO 2 )- (CO) 2 L] 2 [R = Me, Et, t Bu and Ph; L is pyridine (py) or 3-methylpyridine (mpy)]. Despite the diversity of the carboxylate substituents, the core geometry of the binuclear array proved remarkably resistant to any form of systematic substituent variation across the array. A counterpart complex, with an electronically more signi®cantly varied substituent (R = CF 3 and L = py), (II), has since been crystallized and examined structurally as an extension of the series, and is reported here; also reported is the structure of a second monoclinic polymorph, (I), this time in space group P2 1 /c rather than C2/c, of the previously studied Ph/py complex. Both (I) and (II) were obtained as unsolvated crystals; a further phase of the Ph/py complex is a toluene hemisolvate (triclinic, P 1, Z = 2; Xu & Sasaki, 1999). There are no coor- dinates in the Cambridge Structural Database (Allen, 2002) deposition for this form; the limited data in that report, in particular, the RuÐRu distance of 2.6809 (7) A Ê and the mean RuÐO distance of 2.121 (4) A Ê , agree with the following comments. In both phases of (I), [Ru(PhCO 2 )(CO) 2 (py)] 2 , one binuc- lear molecule, devoid of crystallographic symmetry, comprises the asymmetric unit of the structure. The geometric para- meters of the present phase (Table 1 and Fig. 1) are generally similar to those of the other phase insofar as bond lengths and angles are concerned. There is a signi®cant difference, however, in the dihedral angle between the pair of C 2 O 2 carboxylate planes, the present value of 78.20 (7)  being more nearly comparable to the values of 67.5 (1), 71 (1)/76 (1), 73.1 (4) and 70.8 (1)  reported previously for the analogues in which R/L is Me/py, Me/mpy (two molecules), Et/py and t Bu/ py, all of which are very much less than the value found for the C2/c Ph/py phase [84.9 (2)  ]. The molecule of the present phase has quasi-mm symmetry, the pyridine ligands lying in one putative mirror plane (`upright'), the other plane containing the CÐC bonds of the carboxylates and the RuÐ Ru bisector; this symmetry is also compatible with the pyri- dine ligands lying normal to the ®rst mirror plane (`¯at'), as reported for the Me/py adduct. The previous Ph/py (C2/c) molecule is unusual in that the pyridine ligands have a one- upright/one-¯at combination, reducing the putative molecular symmetry to m. It seems unlikely that this conformation (rather than, for example, `packing forces') is responsible for the considerably greater splaying of the carboxylate ligands in the less symmetrical form; the disparity in, for example, the angles about the two Ru atoms is less than that in the present, more symmetric, array. The C 6 /CO 2 interplanar dihedral angles of the carboxylate ligands of the C2/c form are 15.7 (2) and 10.2 (1)  [cf. the present values of 4.62 (7) and 6.55 (6)  ]; the Ru-atom deviations from the CCO 2 planes are 0.148 (6) and 0.131 (6) A Ê for atom Ru1, and 0.228 (6) and 0.254 (5) A Ê for atom Ru2. In the tri¯uoroacetate/pyridine counterpart, [Ru(F 3 CCO 2 )- (CO) 2 (py)] 2 , (II), one complete binuclear neutral molecule again composes the asymmetric unit of the structure; the putative symmetry is again quasi-mm, with a pair of `upright' pyridine ligands. That symmetry is broken, however, by the pair of CF 3 substituents, whose mutual orientations are `geared'. Again, much of the geometry of the core of the dimer (Table 2 and Fig. 2) is similar to that found in the related complexes. Worthy of note as being different, however, are the RuÐO distances, with a mean value of 2.144 (8) A Ê [cf. the mean value of 2.13 (1) A Ê for the C2/c and P2 1 /c benzoates], in Acta Cryst. (2003). C59, m537±m539 DOI: 10.1107/S0108270103023709 # 2003 International Union of Crystallography m537 metal-organic compounds Acta Crystallographica Section C Crystal Structure Communications ISSN 0108-2701