We report a photoelectron spectroscopy (PES) and theoretical study on a series of transition metal halide complexes: FeX4- and MX3- (M=Mn, Fe, Co, Ni, X=Cl, Br). PES spectra were obtained at two photon energies (193 and 157 nm), revealing the complicated electronic structures of these metal complexes and their variation with the ligand-field geometry and metal center substitution. Density functional calculations were carried out to obtain information about the structures, energetics, and molecular orbitals of the metal complexes and used to interpret the PES spectra. For the tetrahedrally coordinated ferric complexes (FeX4-), the PES data directly confirm the "inverted level scheme" electronic structure, where the Fe 3d electrons lie below those of the ligands due to a strong spin-polarization of the Fe 3d levels. For the three-coordinate complexes (MX3-), the calculations also revealed strong spin polarizations, but the molecular orbital diagrams present a "mixed level scheme," in which the ligand orbitals and the Fe 3d majority spin orbitals are spaced closely in the same energy regions. This "mixed level scheme" is due to the larger splitting of the 3d orbitals in the stronger D-3h ligand field and the smaller spin polarizations of the divalent metal centers. The calculations show that the metal 3d orbitals are stabilized gradually relative to the ligand orbitals from Mn to Ni in the tri-halide complexes consistent with the PES spectral patterns. (C) 2003 American Institute of Physics.