过渡金属络合分子二步自旋转换量子理论

Quantum theory of two-step spin-transition for transition-metal complex molecules

本文建立了d4,6络合分子二步自旋转换的量子理论.模拟计算得出了如下几点结论:①低对称畸变是d4,6络合分子可以发生二步自旋转换的必要条件;②对于具有任何一种低对称性的任何一种d4,6络合物存在这种低对称畸变的一个临界值,当畸变小于临界值时只可发生一步自旋转换,即HS LS自旋转换;而当畸变大于临界值时则可以发生二步自旋转换,即HS MS和MS LS自旋转换.二步自旋转换导致磁矩曲线上产生两个拐点,与两个自旋转换点相对应,其间的Dq-距离ΔDq随畸变量的增加而增加;③当畸变量等于临界值时络合分子的两个自旋转换点相重迭,即与自旋转换有关的HS、MS、LS三个能级交于一点,这形成一个自旋三相点,它代表一个由三种自旋相形成的临界状态.因此,自旋三相态或许是可以实现的;④低对称畸变将使高自旋磁矩明显增大,可以高于6.10μB.这表明,通常用实验磁矩数据来判断络合物中心为何种离子的方法是不可靠的;⑤低自旋磁矩可以小于1但显大于0,表明在络合分子中过渡金属离子的轨道角动量未完全碎灭的情况.

A quantum theory for describing two-step-spin-transitions of d^4＇6 complex molecules has been established on the basis of the unified-crystal-field-coupling （UCFC） theory. By simulating calculation we may conclude the following points： ① A low symmetry distortion is a necessary inner condition for any d^4＇6 complex to oeeur a two-step-spin-transition.② For any d^4＇6 complex having any low symmetry distortion there exists a critical value of such a low symmetry distortion： the complex having its distortion smaller than the critical value can and only can occur a one-step-spin-transition, i.e. a high-spin←→low-spin transition; The one having its distortion greater than the eritieal value can consequentially oeeur a two-step-spin-transition, i. e. a high-spin←→intermediate-spin transition together with a intermediate-spin←→low-spin transition, leading to two inflexions on the magnetic moment curve corresponding to the two transition points, between them the Dq-distanee △Dq will increase with increasing the distortion. ③ As for the complex having its distortion equal to the critical value, its two transition points will supertpose into one, i.e. the three lowest levels, a high-spin-, a intermediate-spin- and a low-spin-level, will intersect at one point. This is a spin-three-phasespoint, representing a critical state formed by three spin-phases. So it is predicted that such critical state will be realized in principle. ④ A low symmetry distortion increases markedly the magnetic moment of high-spin states. For example, the high-spin magnetic moment of regular octahedron d6 complexes is 5.64 μB, however a sufficient tetragonal or rhombic distortion can make it higher than 6.10μB. In view of the experimental magnetic moment is 5.10-5.70 μB for d6 configuration Fe^2＋ ion and 5.65-6.10μB for d5 configuration Mn^2＋ ion, this theoretical result indicates that the method for estimating what is the central ion of a complex only according to the magnetic moment experiment is inauthentic. ⑤ The low spin magnetic moment of all octahedron d^6 complexes can smaller than 1 but evidently greater than 0. This result means that the orbital angle moment does not completely vanish for the transition-metal Fe（Ⅱ） ion in complex molecules.