Editing Content :: Admin Scraper

Chapter

Content type

Position

Value

Example 5.7 The spin only magnetic moment of [MnBr ] 2– is 5.9 BM. Predict the 4 geometry of the complex ion ? 2+ Solution Since the coordination number of Mn ion in the complex ion is 4, it will be either tetrahedral (sp hybridisation) or square planar (dsp 2 hybridisation). But the fact that the magnetic moment of the complex ion is 5.9 BM, it should be tetrahedral in shape rather than square planarbecauseofthepresenceoffiveunpairedelectronsinthedorbitals. 5.5.3 Limitations While the VB theory, to a larger extent, explains the formation, structures of Valence and magnetic behaviour of coordination compounds, it suffers from Bond the following shortcomings: Theory (i) It involves a number of assumptions. (ii) It does not give quantitative interpretation of magnetic data. (iii) It does not explain the colour exhibited by coordination compounds. (iv) It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds. (v) It does not make exact predictions regarding the tetrahedral and square planar structures of 4-coordinate complexes. (vi) It does not distinguish between weak and strong ligands. 5.5.4 Crystal The crystal field theory (CFT) is an electrostatic model which considers Field the metal-ligand bond to be ionic arising purely from electrostatic Theory interactions between the metal ion and the ligand. Ligands are treated as point charges in case of anions or point dipoles in case of neutral molecules. The five d orbitals in an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. This degeneracy is maintained if a spherically symmetrical field of negative charges surrounds the metal atom/ion. However, when this negative field is due to ligands (either anions or the negative ends of dipolar molecules like NH 3nd H O) 2n a complex, it becomes asymmetrical and the degeneracy of the d orbitals is lifted. It results in splitting of the d orbitals. The pattern of splitting depends upon the nature of the crystal field. Let us explain this splitting in different crystal fields. (a) Crystal field splitting in octahedral coordination entities In an octahedral coordination entity with six ligands surrounding the metal atom/ion, there will be repulsion between the electrons in metal d orbitals and the electrons (or negative charges) of the ligands. Such a repulsion is more when the metal d orbital is directed towards the ligand than when it is away from the ligand. Thus, the d x y and d z orbitals which point towards the axes along the direction of the ligand will experience more repulsion and will be raised in energy; and the d , d and d orbitals which are directed between xy yz xz the axes will be lowered in energy relative to the average energy in the spherical crystal field. Thus, the degeneracy of the d orbitals has been removed due to ligand electron-metal electron repulsions in the octahedral complex to yield three orbitals of lower energy, t2g set and two orbitals of higher energy, e sgt. This splitting of the 131 Coordination Compounds Reprint 2026-27