9.5 Bonding in Coordination Compounds 19 the orbitals now have different energies. In an isolated gaseous metal ion, all the five d orbitals have the same energy, and are termed degenerate. If a spherically symmetrical field of negative charges surrounds the metal ion, the d orbitals remain degenerate. However, the energy of the orbitals is raised because of repulsion between the field and the electrons on the metal. In most transition metal complexes, either six or four ligands surround the metal, giving octahedral or tetrahedral structures. In both of these cases, the field produced by the ligands is not spherically symmetrical. Thus the d orbitals are not all affected equally by the ligand field.
Thus, under the influence of the ligands, the five degenerate d orbitals of the metal ion will split into two groups of orbitals of different energies. This effect is known as crystal field splitting or energy level splitting.
Crystal Field Splitting in Octahedral Coordination Entities In an octahedral complex, the coordination number is 6. The central metal ion is at the center and the six ligands occupy the six corners of the octahedron. The phenomenon of splitting of d orbitals in an octahedral complex is illustrated in Fig. 9.6. dx 2 − y 2 dz 2 (eg)
0.6∆ο ∆
Energy
State II 0.4∆ο State I
dxy dyz dzx (t2g ) State III
Figure 9.6 Splitting of d orbitals in an octahedral complex. In the free metal ion, all the five d orbitals are degenerate (State I, Fig. 9.6). When the six ligands approach the central metal cation along the axes, they exert an electrostatic force of repulsion on the outer d electrons; that is, the d electrons are repelled by the lone pairs of the ligands. This repulsion raises the energy of the degenerate d orbitals to give five excited degenerate orbitals (State II, Fig. 9.6). Since the lobes of d 2 and d 2 2 orbitals (eg set of orbitals) lie directly in the path of the z x −y approaching ligands, the electrons in these orbitals experience greater repulsion exerted by the electron clouds of the ligands than that experienced by the electrons in the dxy, dyz and dzx orbitals (t2g set of orbitals), which are directed in space between the x, y and z axes. Hence, under the influence of the approaching ligands, the orbitals d 2 2 and d 2 are raised in energy, whereas the x −y z orbitals dxy, dyz and dzx are lowered in energy relative to the excited d levels (State III, Fig. 9.6). The separation of five d orbitals into t2g and eg sets of different energies is known as crystal field splitting. The eg set, which is of higher energy, is doubly degenerate; whereas the t2g set, which is lower in energy, is triply degenerate. The energy difference between eg and t2g sets is denoted by ∆o (the subscript o stands for octahedral). The magnitude of ∆o depends on the field strength of the ligand as well as on the metal ion. ∆o is called the crystal field stabilization energy (CFSE).The energy of the two eg orbitals increases by 0.6∆o (3/5∆o) and that of t2g orbitals is lowered by 0.4∆o (2/5∆o).The magnitude of crystal field splitting depends on a number of factors, the most important amongst which is the nature of the ligand. If it is easier for a ligand to approach metal atom and interact with it, then the extent of crystal field splitting is high. The ligands that cause only a small degree of crystal field splitting are called weak ligands, whereas the ligands that cause a large degree of crystal field splitting are called strong ligands. The strong ligands give a higher value of ∆o, and weak ones give a lower value. Some common ligands arranged in the increasing order of their splitting power are as follows: I− < Br − < S2− < Cl− < NO3− < F − < OH− < EtOH < (COO − )2 < H2 < EDTA < NH3 < ethylenediamine < diphenyl < o-phenanthroline < NO2− < CN− < CO
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