Europium-151 Mössbauer Effect Study of Several Organoeuropium(II) Complexes

The europium-151 Mössbauer spectra of several organoeuropium(II) compounds have been measured at 77 and 4.2 K. All the compounds give isomer shifts of ca. -12 to -13 mm/s, typical of ionic europium(II). For (C5Me5)2Eu, (C5Me5)2Eu(THF), and (C5Me5)2Eu(THF)(Et2O), broad absorptions were observed, arising from spherical paramagnetic relaxation at a rate comparable to the Mössbauer time scale of 9.7 × 10-9 s. The relaxation rates are 490, 450, and 420 MHz, respectively, at 4.2 K, and the values for (C5Me5)2Eu(THF) and (C5Me5)2Eu(THF)(Et2O) increase to 635 and 650 MHz at 77 K as a result of greater coupling of the spin with the lattice vibrations. For [(C5Me5)Eu(THF)2(-I)]2 the spectrum corresponds to a relaxation rate of 1710 MHz at 4.2 K. [(C5Me5)2Eu(THF)]n shows a symmetric spectrum that is close to the fast relaxation limit with an estimated relaxation rate of 5580 MHz. EuI2(THF)2 shows a narrow line width spectrum with a quadrupole interaction of -12.9 mm/s and an asymmetric parameter of 0.58 at 4.2 K. The rate of paramagnetic relaxation correlates well with the inverse of the distance between Eu(II) ions, suggesting that the relaxation occurs via a spin-spin interaction. The magnetic susceptibility of (C5Me5)2Eu, measured between 2 and 320 K, shows it to be paramagnetic in this temperature range with a moment of 6.99 B and a Curie-Weiss temperature of 3.28 K