C19H12M0O5R11, monoclinic, Pl2\ln (no. 14), a = 10.0098(2) A, b = 8.7334(2) A, c = 19.4450(6) A, β = 97.046(1)°, V= 1687.0 A, Z = 4, RgJF) = 0.023, wAreffF) = 0.057, T= 173 K. Source of material Ind(CO)3Mo-Ru(CO)2Cp was synthesized according to a modified procedure [1]. 3.0 ml (4.8 mmol) of a 1.6 Μ solution of nBuLi in hexanes were added dropwise to 0.6 ml (5.2 mmol) indene in 30 ml THF at -78 °C. After stirring for 3 h at RT, 1.056 g (4.0 mmol) Mo(CO)6 were added and the mixture refluxed for 24 h until the formation of NaMo(CO)3lnd was complete (IR control). A solution of 1.2 g (4.0 mmol) CpRu(CO)2Br [2] in 25 ml THF was added dropwise at RT, and it was refluxed until the starting materials were consumed (30 h, IR control). The solvents were removed at high vacuum and the residue extracted with 3 χ 50 ml hexane. The extracts were combined, filtered and the target compound crystallized at -20 °C. Slow recrystallization (flask in sealed dewar) by cooling from RT to 2 0 °C yielded crystals suitable for X-ray single-crystal diffraction. Discussion The title compound consists of a Mo(CO)3lnd and a Ru(CO>2Cp fragment, linked by an unsupported M-M' bond. The observed Mo—Ru bond length of3.0048(3) A is in the expected molybdenum-ruthenium single bond range [3]. The five caibonyl ligands are all essentially terminal, reflected by M(l)-M(2)-CO angles not smaller than 68.42(8)° and M-C-O angles larger than 171.9(2)°. However, in order to minimize oxygen contacts, two of the Mo-CO ligands are slightly more bent than the other carbonyl groups, and the short non-bonded contacts between C1 and C3 as well as between C2 and C4 (3.05 A and 3.07 A, respectively) are typical for this type of complex [1,4]. At room temperature, the carbon NMR spectra reveals a single resonance for all caibonyl groups, which indicates a facile and rapid CO interchange in and between the two metal moieties in solution. The >7cyclopentadienyl ligand adopts a centric co-ordination to the ruthenium, is located in trans position to the indenyl ligand relative to the Mo-Ru vector, and exhibits structural properties known from related compounds [3,5]. The bonding of the hydrocarbon fragment to the molybdenum is an intriguing feature of this complex because of the possibility of distortions toward an η co-ordination mode of the five-membered ring of the indenyl ligand. The corresponding Mo—C distances vary significantly between d(Mo—C2) = 2.315(3) A, d(Mo—C3) = 2.378(3) A, d(Mo—C8) = 2.427(3) A, d(Mo—C9) = 2.393(3) A, and d(Mo—C10) = 2.358(3) A. However, the average Μ—C(bridgehead) bond length is by only 0.058 A longer than the average Μ—C bond distances of the remaining three carbon atoms of the five-membered ring and the indenyl ligand is strictly planar, so the target compound features an essentially undistorted ηindenyl molybdenum fragment [6]. The slight tilting of the indenyl ligand, resulting in the elongation of the Mo—C8 bond is assigned to the trans influence of the CO-ligand trans to the Mo—C8 bond [7,8]. Table 1. Data collection and handling