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镧系离子的4f轨道在空间上被5s~25p~6轨道所屏蔽,因此不太被配位场所影响,所以以前认为稀土化合物是属离子型化合物.但大量事实表明.屏蔽并不是完全的,配位场的作用仍不可忽略,有一些镧系化合物并不是纯离子型的,而是有一定程度的共价性.这可从它们的f-f跃迁的光谱谱线在配位场的作用下发生位移而得到证明.在配位场的作用下,4f轨道仍有一点伸展扩大,与5d轨道混合而参与成键,从而有一定程度的共价性.由于这种电子云的扩大效应(nephelauxetic effect),使电子间的互斥减小,从而减少了多重项之间的间距,使谱线发生向长波的位移(红移).因此,可根据谱线的红移作为镧系离子与配位体之间的共价程度的测量.由于目前对气态的镧系自由离子的吸收光谱还没有做过很多工作,缺乏数据,因此,目前只能相对于镧系的水合离子来测定镧系化合物的相对谱线位移.
4f orbital of lanthanide ions are spatially confined by 5s ~ 25p ~ 6 orbitals and therefore are not affected by coordination sites, so rare earth compounds were previously considered to be ionic compounds, but a large number of facts show that shielding is not complete, The role of the coordination field can not be ignored, and some lanthanide compounds are not purely ionic but rather have a degree of covalence, which can occur from the spectral line of their ff transition under the action of a coordination field It is proved that the 4f orbit is still slightly extended and expanded due to the coordinating field and has a certain degree of covalence due to its mixing with the 5d orbit because of the nephelauxetic effect ), The mutual exclusion between electrons is reduced, which reduces the spacing between multiple items and shifts the spectrum to longwave (redshift). Therefore, according to the spectral redshift as the lanthanide ions and coordination Since the current absorption spectra of the free lanthanide ions in gaseous state have not done a lot of work, lack of data, so far, only lanthanide hydrate ion relative to the determination of lanthanide compounds Relative spectral line displacement.