On the basis of analytical data and the following arguments the new complex is assigned the structure ( 3 ) in which two tetracarbonylmanganese fragments are symmetrically bridged by two ethoxycarbonylcarbyne ligdnds. I ) In the 'H-NMR spectrum (60 MHz; 33Β°C) the methylene protons appear as a quartet at r=5.64, the methyl protons as a triplet at ~= 8 . 6 5 in the intensity ratio 2:3 (CDzClz; internal TMS). No hindrance to free rotation of the ester groupsabout the C-C axes can be observed in the 'H-NMR spectrum down to -70Β°C. 2) In the mass spectrum (70eV; direct sample injection at 120Β°C: 7;,=40--60"C) the molecular ion M (m/e=504) is accompanied, inter aliu, by the complete fragmentation sequence M -n C O (n= 1 to 8). Hence ( 3 ) contains eight carbonyl groups bound to the metal. 3) In accord with group-theoretical predictions for a molecule of point group DZhr the IR spectrum (n-hexane) contains four bands in the range of the vco fundamental vibration (B3Ja), 2076m: B1,, 2 0 0 4 ~s ; B3,(b), 1 9 9 3 ~-s ; Blu, 1956cm-'vs) whose positions and intensities resemble those of the IR absorptions of the isostructural complexes [(CO),MX], (M =Mn, Tc, Re; X=C1, Br, I)[31.
4 ) The numbers of calculated and observed Raman active vco fundamental vibrations agree: The four emission bands (B?,, 2 0 9 0 ~s ; Bt,, 1999vs; A,(a), 1954m; A,(b), 1935cm-I m) are not coincident with the K R absorptions owing to the selection rule applicable to ( 3 ) owing to centrosymmetry. 5) The highly symmetric charge distribution resulting for the proposed structure is confirmed by the virtual absence of a dipole moment in (3) (p = 0.18 i.0. IS Debye; cyclohexane, 25 "C). 6) The experimentally detected diamagnetism corresponds to the noble gas configuration in (3). Each one of the ethoxycarbonylcarbyne bridges acts as a three-electron Iigand and thus resembles the monodentate (ary1)alkylcarbyne ligandsIJ1.
Di-p-e/ hou!,curbon!.lcurb!,nebis( tetracurbon!~lmunganese) ( 3 ) :
All operations are performed in an inert atmosphere (rJz) using dry (Na/K) nitrogen-saturated solvents. Compounds ( 1 ) ( 1.28 g, 3 mmol) [51 and (2) (1.65 g, 6mm0l) ['~ are refluxed in diethyl ether (200ml) for 10 h. After removal of the solvent, the benzene-soluble portion of the crude product is filtered (G4 frit). The filtrate is concentrated by evaporation and then chromatographed on silica gel (Merck 7734, activity grade I; water-cooled column). Elution with n-hexane affords a little ( 2 ) as light yellow zone. With n-hexane/benzene ( 5 : l ) an orange-yellow zone migrates which affords fine needles of (3) on concentration and repeated crystallization (n-pentane, -85 "C); yield 570 mg (38 'x).