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 for mula (CO)(3)Cr(mu-Ind)RhL(2) (L = CO, ethylene; L(2)= COD, COT, NBD) h ave two possible conformers, one with the metals at antipodal sides of the condensed bicyclic polyene (antarafacial, anti) and the other wit h both metals on the same side (cofacial, syn). These systems can be c onsidered to contain 34 valence electrons by assuming that Ind is capa ble of donating all of its ten rr electrons to the metals. A qualitati ve MO analysis, based on EHMO calculations, is presented. The anti con former is compared to the homometallic 34e species, namely [CpRu(mu-In d)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, wh ich thus become formally saturated (36e). This argument is extended to the anti Cr-Rh derivative in which a critical role is played by an em pty FMO of the (CO),Rh(I) fragment with pi(11), symmetry (i.e. lying i n the molecular mirror plane). The latter, in spite of the large pi co contributions, has good acceptor capabilities at the metal. The calc ulations suggest that the so-called indenyl effect, namely the increas ed 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) F MO. In the series (eta(5)-C5H5)Rh(CO)(2), (eta(5)-C7H9)Rh(CO)(2) and e ta(5)-[(CO)(3)CrC7H9)]-Rh(CO)(2) the progressively reduced donor capab ilities of the cyclic polyene toward the (CO)(2)Rh(I) fragment induce a larger electrophilicity in rhodium, in good agreement with the exper imental data on substitution reactivity. Finally, the electronic featu res 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 saturat ion. In this case, the limited propensity of the complex toward any CO substitution reactions is likely attributable to the hindered mobilit y of the (CO)(2)Rh(I) fragment.