Key Words: Tetraindane(4) / Indium@) I Mn-Mn bond / Indium alkyl bridge Tetrclhedra-tetrakis[tris(trimethylsilyl)methyl]tetraindane(4)(1) reacts with decacarbonyldimanganese(0) to yield the bright red crystalline title compound 2, in which two and a planar MnzInz molecular center. carbonyl ligands are replaced by two InR fragments. The crystal structure determination of 2 shows two M n ( C 0 ) 4 groups (Mn-Mn 313.7 pm) bridged by two monoalkylindium units Recently, we succeeded in synthesizing and determining the crystal structure of the alkylindium(1) compound I I I~[ C ( S~M~~)~]~ (1) with a nearly undistorted tetrahedral In4 skeleton in the solid state"], which in contrast to the Ga4 analog[*] remains tetrameric in dilute benzene solutions. In the course of systematic investigations of the chemical properties of 1[1,3] we found that the monomeric In-C(SiMe& fragment is able to substitute carbonyl ligands in transition metal carbonyls. In this paper we report on the reaction of 1 with Mn2(CO)lo. Reaction of In4[C(SiMe3)3]4 with Mnz(CO)loMixtures of 1 and Mnz(CO)lo in n-hexane remain unchanged at room temperature for several days, while in boiling hexane the characteristically deep violet color of 1 disappears within two or three hours, depending on the excess of the manganese carbonyl used. A dark red solution is formed, from which after concentration and recrystallization dark red, very fine needles of the title compound 2 are isolated in 60% yield. The molar mass, cryoscopically determined in benzene solution, and the mass spectrum with a very characteristic isotopic pattern show that two carbonyl groups of the Mn2(CO)lo molecule are replaced by two monoalkylindium fragments [eq. The trimethylsilyl groups of 2 show one resonance in both the 'H-and I3C-NMR spectra. Although the molecular structure of 2 exhibits two chemically different carbonyl groups, only one I3C resonance is observed even at -65°C with the chemical shift 8 = 218. The assignment of the a-carbon atom of the tris(trimethy1si-1yl)methyl group in the 13C-NMR spectrum is not beyond doubt: there is only one further signal at 6 = 51.7 lying more than 25 ppm downfield with respect to the region normally observed for carbon atoms bound to indiumL61. The location of this carbon atom in the nodal plane of the p orbital, which is perpendicular to the molecular plane and can accept x electron density by backdonation, might be responsible for this unusual shift. Figure 1 shows the molecular structure of 2. Molecule 2 contains an Mn -Mn bond bridged by two monoalkylindium fragments. Crystal Structure of 2Whereas several compounds with bridged Mn-Mn bonds are known ['], only a few examples exhibit a bridging third main group element[8-'01. Indium as a bridging atom is mostly tetracoordinatedL9I, a lower coordination number is found in a dimanganese compound bridged by an In-Mn(CO)5 group[l0I. To our knowledge bridging alkylindium units with tricoordinated In atoms like in 2 have previously not been described. The synthesis according to eq. (1) therefore ope...