Cofacial and antarafacial indenyl bimetallic isomers: a descriptive MO picture and implications for the indenyl effect on ligand substitution reactions

The hetero-bimetallic Ind complexes (Ind = indenyl anion) with the formula (CO)(3)Cr(mu-Ind)RhL(2) (L = CO, ethylene; L(2)= COD, COT, NBD) have two possible conformers, one with the metals at antipodal sides of the condensed bicyclic polyene (antarafacial, anti) and the other with both metals on the same side (cofacial, syn). These systems can be considered to contain 34 valence electrons by assuming that Ind is capable of donating all of its ten rr electrons to the metals. A qualitative MO analysis, based on EHMO calculations, is presented. The anti conformer is compared to the homometallic 34e species, namely [CpRu(mu-Ind)RuCp](+). Here, the pseudo-symmetrical relationship between the two identical metal fragments strongly suggests that one Ind sigma-bonding MO donates part of its electron density to each of the two metals, which thus become formally saturated (36e). This argument is extended to the anti Cr-Rh derivative in which a critical role is played by an empty FMO of the (CO),Rh(I) fragment with pi(11), symmetry (i.e. lying in the molecular mirror plane). The latter, in spite of the large pi co* contributions, has good acceptor capabilities at the metal. The calculations suggest that the so-called indenyl effect, namely the increased rate of substitution of one CO ligand by another sigma donor (e.g. a phosphine), although a structurally dynamic process, also depends on the electron density which accumulates on the aforementioned pi(11) FMO. In the series (eta(5)-C5H5)Rh(CO)(2), (eta(5)-C7H9)Rh(CO)(2) and eta(5)-[(CO)(3)CrC7H9)]-Rh(CO)(2) the progressively reduced donor capabilities of the cyclic polyene toward the (CO)(2)Rh(I) fragment induce a larger electrophilicity in rhodium, in good agreement with the experimental data on substitution reactivity. Finally, the electronic features of the cofacial conformer (CO)(3)Cr(mu-Ind)Rh(CO)(2) are examined. In spite of the long Cr-Rh separation ( > 3 Angstrom) the graphically illustrated MO analysis indicates a direct, heterodox, intermetallic linkage, which helps the metals to achieve a formal electronic saturation. In this case, the limited propensity of the complex toward any CO substitution reactions is likely attributable to the hindered mobility of the (CO)(2)Rh(I) fragment.