X-RAY ABSORPTION SPECTROSCOPIC STUDY OF THE TEMPERATURE AND PRESSURE DEPENDENCE OF THE ELECTRONIC SPIN STATES IN SEVERAL IRON(II) AND COBALT(II) TRIS( PYRAZOLYL)BORATE COMPLEXES

An iron and cobalt K-edge X-ray absorption study has been undertaken, at 295 and 77 K, to investigate the electronic spin states of Fe[HB(pz)3]2, 1, Fe[HB(3,5-(CH3)2pz)3]2, 2, Fe[HB(3,4,5-(CH3)3pz)3]2, 3, Co[HB(pz)3]2, 4, Co[HB(3,5-(CH3)2pz)3]2, 5, and Co[HB(3,4,5-(CH3)3pz)3]2, 6, where pz is the 1-pyrazolyl moiety. Between 295 and 77 K complex 2 shows a spin-state crossover whereas 1 and 3−6 remain low-spin and high-spin, respectively. The spectra show a clear difference, in both the relative intensities and the relative energies, of the metal 1s to 4p electronic transition in the high-spin and the low-spin states. An analogous study of Fe[HB(pz)3]2, 1, between 295 and 450 K reveals that it undergoes a gradual reversible spin-state crossover to the high-spin state above ca. 360 K. The high-pressure room-temperature XANES spectra of 1 indicate that it remains low-spin between ambient pressure and 90 kbar, whereas complex 2 shows the expected spin-state crossover between zero and ca. 30 kbar and is low-spi...