PRESSURE-INDUCED SPIN-STATE CROSSOVERS IN 6-COORDINATE FE(II)L(N)L'(M)(NCS)(2) COMPLEXES WITH L=L' AND L-NOT-EQUAL-L' - A XANES INVESTIGATION

The pressure-induced iron(II) high-spin (HS) --> low-spin (LS) convers ion has been investigated, by near-edge X-ray absorption (XANES) spect roscopy at room temperature, in a number of parent six-coordinate comp lexes of the type FeL(n)L'(m)(NCS)(2), viz. Fe(phen)(2)(NCS)(2) form I (1) and form II (2), Fe(py)(2)bpym(NCS)2 (3), Fe(py)(2)phen(NCS)(2) ( 4) and Fe(py)(4)(NCS)(2) (5), where phen = 1,10-phenanthroline, py = p yridine, and bpym 2,2'-bipyrimidine. The spectra of the two spin isome rs are interpreted. For compounds 1-4, known to exhibit thermally-indu ced spin transitions, n(LS) vs P plots (n(LS) = LS fraction, P = press ure) are centered around P-c = 0.80 (1), 0.65 (2), 1.00 (3) and 1.55 ( 4) GPa. For 5, which retains the HS form at any temperature under atmo spheric pressure, the P-c value is much higher than the previous ones, lying in the range 5-7 GPa. After pressure was released, the spectra were found to be quite similar to those obtained before pressure was a pplied. The P-c values of 1-4 were analyzed in terms of spin-transitio n temperatures (T-c), entropy (Delta S) and crystal volume (Delta V-SC ) variations associated with the spin change, crystal structures, and volumic compressibility coefficients (k(v)), insofar as these data wer e available. The alteration of the spin-conversion development, when s ubjecting a compound (here 2) several times to increasing then to decr easing pressures, was ascribed to a progressive decrease in the number of crystal defects (vacancies in particular).