ELECTRONIC-STRUCTURE AND PHOTOELECTRON-SPECTROSCOPY OF D(8) RHODIUM INDENYL COMPLEXES

The parent indenyl compounds (eta-C9H7)RhL2 (1a-c), trimethylindenyl complexes (eta-1,2,3-Me3C9H4)RhL2 (2a-c), and permethylindenyl complexes (eta-C9Me7)RhL2 (3a-c) (a, L = eta-C2H4; b, L2 = eta4-1,5-C8H12; C, L = CO) have been prepared and examined by photoelectron spectroscopy (PES). Extended Huckel molecular orbital calculations were also carried out on (eta-C9H7)RhL2 (L = eta-C2H4, CO). The electron donating ability of the three indenyl ligands, based on the ionization energies of the nonbonding rhodium centered orbitals, is in the order (eta-C9Me7) > (eta-1,2,3-Me3C9H4) > (eta-C9H7). The (eta-C9H7) (ligand is a stronger donor than (eta-C5H5). Somewhat lower values of nu(CO) in the IR spectra and significantly smaller measured barriers to ethylene rotation in the indenyl complexes vs their cyclopentadienyl analogues can be traced to weaker Rh-L pi-back-bonding, mainly d(xz), which is also the acceptor orbital for indenyl pi4. The major factor determining the relative amounts of back-donation is therefore the splitting of the indenyl pi5 and pi4, the two highest lying filled pi-orbitals of the indenyl anion, vs the degenerate el set of eta-C5H5 which lies between pi4 and pi5 in energy. A rationale for increased S(N)1 substitution rates and catalytic activity of indenyl vs cyclopentadienyl complexes is provided.