Electronic Structure of cis-Dimethylplatinum(II) Complexes from UV Photoelectron Spectra and SCF–MS–Xα Calculations

UV photoelectron spectra of cis-[PtMe₂L₂] [L = PMe₃, PMe₂Ph, PMePh₂, PEt₃, P(NMe₂)₃, PMe(OEt)₂, P(OMe)₃, P(OEt)₃, AsMe₃, CNMe, CNPh; L₂ = Me₂NCH₂CH₂NMe₂ (TMED), 1,5-cyclooctadiene (COD), norbornadiene (NBD)] are reported, as well as MS–Xα calculations on the model compounds cis-[PtMe₂(EH₃)₂] (E = N, P, As), cis-[PtMe₂(C₂H₄)₂], and cis- [PtMe₂(CNH)₂], The results from these calculations and spectral information, such as resolved vibrational fine structures and ligand shift effects, have allowed the assignment of the lower energy ionizations to specific molecular orbitals. The two molecular orbitals with lowest ionization energies are assigned, in all cases, to Pt–ligand σ orbitals, and these are followed by four MO's that have mostly nonbonding Pt 5d character. The σ-donor ability of a ligand L is reflected in the first ionization energy and decreases in the order L = P(NMe₂)₃ > TMED > PMePh₂ ~ PMe₂Ph > PEt₃ > AsMe₃ ~ PMe₃ > PMe(OEt)₂ > P(OEt)₃ > P(OMe)₃ > COD > NBD. The π-acceptor ability is reflected by the energy separation of the Pt d_xy and d_xy (axes defined by C_2v symmetry) and falls in the order PMePh₂ ~ PMe₂Ph ~ P(OMe)₃ ~ P(OEt)₃ ~ PMe(OEt)₂ ~ P(NMe₂)₃ > AsMe₃ ~ PMe₃ ~ PEt₃ ~ COD ~ NBD > TMED. Both isocyanide ligands CNMe and CNPh, which cannot be arranged into the above series, have comparable σ-donor ability, and the latter has stronger π-donor and π-acceptor properties. Some typical chemical processes are also discussed on the basis of the established σ/π series.