Electronic Relaxation Phenomena Following 57Co(EC)57Fe Nuclear Decay in [MnII(terpy)2](ClO4)2·1/2H2O and in the Spin Crossover Complexes [CoII(terpy)2]X2·nH2O (X = Cl and ClO4): A Mössbauer Emission Spectroscopic Study‡

The valence states of the nucleogenic 57Fe arising from the nuclear disintegration of radioactive 57Co by electron
capture decay, 57Co(EC)57Fe, have been studied by Mössbauer emission spectroscopy (MES) in the 57Co-labeled
systems:  [57Co/Co(terpy)2]Cl2·5H2O (1), [57Co/Co(terpy)2](ClO4)2·1/2H2O (2), and [57Co/Mn(terpy)2](ClO4)2·
1/2H2O (3) (terpy = 2,2‘:6‘,2‘ ‘-terpyridine). The compounds 1, 2, and 3 were labeled with ca. 1 mCi of 57Co and
were used as the Mössbauer sources at variable temperatures between 300 K and ca. 4 K. [Fe(terpy)2]X2 is a
diamagnetic low-spin (LS) complex, independent of the nature of the anion X, while [Co(terpy)2]X2 complexes
show gradual spin transition as the temperature is varied. The Co(II) ion in 1 “feels” a somewhat stronger ligand
field than that in 2; as a result, 83% of 1 stays in the LS state at 321 K, while in 2 the high-spin (HS) state
dominates at 320 K and converts gradually to the LS state with a transition temperature of T1/2 ≈ 180 K. Variable-temperature Mössbauer emission spectra for 1, 2, and 3 showed only LS-57Fe(II) species at 295 K. On lowering
the temperature, metastable HS Fe(II) species generated by the 57Co(EC)57Fe process start to grow at ca. 100 K
in 1, at ca. 200 K in 2, and at ca. 250 K in 3, reaching maximum values of 0.3 at 20 K in 1, 0.8 at 50 K in 2,
and 0.86 at 100 K in 3, respectively. The lifetime of the metastable HS states correlates with the local ligand field
strength, and this is in line with the “inverse energy gap law” already successfully applied in LIESST relaxation
studies.

对源自放射性57Co通过电子俘获衰变而发生的核裂变所生成的核素57Fe的价态进行了研究。该研究采用穆斯堡尔发射光谱学（MES）方法，在标记有57Co的系统[57Co/Co(terpy)2]Cl2·5H2O（1）、[57Co/Co(terpy)2](ClO4)2·1/2H2O（2）和[57Co/Mn(terpy)2](ClO4)2·1/2H2O（3）中进行（其中terpy代表2,2‘:6‘,2‘-terpyridine）。这些化合物1、2和3分别用约1毫居里的57Co进行标记，并在300 K至约4 K的温度范围内作为穆斯堡尔源进行测试。[Fe(terpy)2]X2是一种反磁性低自旋（LS）络合物，其性质与阴离子X的种类无关，而[Co(terpy)2]X2络合物则随着温度的变化显示出逐渐的磁矩转变。在化合物1中，Co(II)离子感受到的配位场略强于化合物2中的；因此，在321 K时，1中有83%保持在LS状态，而在2中，高自旋（HS）状态在320 K时占主导地位，并逐渐转变为LS状态，其转变温度为T1/2 ≈ 180 K。1、2和3的变温穆斯堡尔发射光谱在295 K时仅显示出LS-57Fe(II)物种。随着温度的降低，由57Co(EC)57Fe过程产生的亚稳态HS Fe(II)物种在1中约100 K时开始增长，在2中约200 K时开始增长，在3中约250 K时开始增长，分别在1中的20 K时达到最大值0.3，在2中的50 K时达到最大值0.8，在3中的100 K时达到最大值0.86。亚稳态HS状态的寿命与局部配位场的强度相关，这与已在LIESST弛豫研究中成功应用的“逆能隙定律”相一致。