Evidence of Spin Crossover Phenomena Deduced from Gas-Phase Photoelectron Spectra of the Bis[tetrakis(pyrazol-1-yl)borato]iron(II) Complex

The gas-phase He I and He II photoelectron spectra of the bis[tetrakis(pyrazol-1-yl)borato]iron(II) complex have been recorded. In striking contrast with the low-spin only 1A1 state so far associated with its d6 ground configuration, the spectral pattern of the d-1 final ion states is almost identical to that produced upon ionization from the 5T2 ground state of the thermally activated high-spin bis[tris(pyrazol-1-yl)borato]iron(II) parent complex. The multiplet onset structure in the photoelectron spectrum suggests the occurrence of the 5T2 ground state and, hence, of spin crossover phenomena. The energies associated with the onset multiplet structure have been used to fit the corresponding Racah equations for the electrostatic energies of the ion states produced upon ionization from the 5T2 ground state. A 1.53-eV ligand field DELTA value has been found. There is indication of a more pronounced trigonal distortion of the present complex with respect to the bis[tris(pyrazol-1-yl)borato]iron(II) parent complex.