Electron Distribution and Bonding in η³-Cyclopropenyl-Metal Complexes

The synthesis of (η³-C₃R₃)Ir(CO)₃ and He I and He II photoelectron spectra of (η³-C₃R₃)Co(CO)₃, (η³-C₃R₃)Ir(CO)₃, and (η³-C₃R₃)Fe(CO)₂(NO) (where R = tert-butyl) are reported. The shifts and splittings in ionization energies with the metal and ligand perturbations in this series, the changes in ionization peak areas as a function of the excitation energy, and Fenske-Hall molecular orbital calculations assist in the assignment and interpretation of the spectra. The cobalt complex reveals three peaks in the low ionization energy region that are primarily metal-based, consistent with the two states of e symmetry and one state of a₁ symmetry from the five d orbitals of a formally d¹⁰ metal in C_3ν symmetry. The spectrum of the iridium complex has five peaks in this region due to large spin-orbit coupling that splits the e symmetry ionizations. The doubly degenerate peaks of the cobalt complex are also expected to be split in the photoelectron spectrum of the iron-nitrosyl complex due to the lowering to C_s symmetry. Only four distinct peaks are seen for the iron-nitrosyl complex, with two ionization bands at higher energy being merged in a broad envelope. The ionization cross-sections indicate that the η³-bound C₃R₃ ligand in these complexes is best described formally as a cation with a large amount of mixing and backbonding from the metal d_π orbitals to the e_π* orbitals of the cyclopropenyl ring. This is compared to the NO⁺ ligand.