|
The nephelauxetic effect is a term used in the physical chemistry of transition metals. It refers to a decrease in the Racah interelectronic repulsion parameter, given the symbol ''B'', that occurs when a transition-metal free ion forms a complex with ligands. The name "nephelauxetic" comes from the Greek for ''cloud-expanding''. The presence of this effect brings out the disadvantages of crystal field theory, as this accounts for somewhat covalent character in the metal-metal interaction. ==Racah parameter== The decrease in the Racah parameter ''B'' indicates that in a complex there is less repulsion between the two electrons in a given doubly occupied metal ''d''-orbital than there is in the respective Mn+ gaseous metal ion, which in turn implies that the size of the orbital is larger in the complex. This electron cloud expansion effect may occur for one (or both) of two reasons. One is that the effective positive charge on the metal has decreased. Because the positive charge of the metal is reduced by any negative charge on the ligands, the ''d''-orbitals can expand slightly. The second is the act of overlapping with ligand orbitals and forming covalent bonds increases orbital size, because the resulting molecular orbital is formed from two atomic orbitals. The reduction of ''B'' from its free ion value is normally reported in terms of the nephelauxetic parameter, β :β= ''B''(complex) / ''B'' (free ion) Experimentally, it is observed that size of the nephelauxetic parameter always follows a certain trend with respect to the nature of the ligands present. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nephelauxetic effect」の詳細全文を読む スポンサード リンク
|