The square planar crystal field
المؤلف:
CATHERINE E. HOUSECROFT AND ALAN G. SHARPE
المصدر:
INORGANIC CHEMISTRY
الجزء والصفحة:
2th ed p 562
19-8-2016
1487
The square planar crystal field
A square planar arrangement of ligands can be formally derived from an octahedral array by removal of two transligands (Figure 1.1). If we remove the ligands lying along the z axis, then the dz2 orbital is greatly stabilized; the energies of the dyz and dxz orbitals are also lowered, although to a smaller extent. The resultant ordering of the metal d orbitals is shown at the left-hand side of Figure 1.2. The fact that square planar d8 complexes such as [Ni(CN)4]2- are diamagnetic is a consequence of the relatively large energy difference between the dxy and dx2-y2 orbitals. Worked example 20.1 shows an experimental means (other than single-crystal X-ray diffraction) by which square planar and tetrahedral d8 complexes can be distinguished.
Fig. 1.1 A square planar complex can be derived from an octahedral complex by the removal of two ligands, e.g. those on the z axis; the intermediate stage is a Jahn–Teller distorted (elongated) octahedral complex.
Fig. 1.2 Crystal field splitting diagrams for some common fields referred to a common barycentre; splittings are given with respect to Δoct. For tetrahedral splitting,
Worked example 1.1 Square planar and tetrahedral d8 complexes d 8 complexes
The d8 complexes [Ni(CN(4]2- and [NiCl4]2- are square planar and tetrahedral respe ctively. Will these complexes be paramagnetic or diamagnetic?
Consider the splitting diagrams shown in Figures 20.8 and 20.10. For [Ni(CN)4]2- and [NiCl4]2- , the eight electrons occupy the d orbitals as follows:
Thus, [NiCl4]2- is paramagnetic while [Ni(CN)4]2- is diamagnetic. Although [NiCl4]2- is tetrahedral and paramagnetic, [PdCl4]2- and [PtCl4]2- are square planar and diamagnetic.
This difference is a consequence of the larger crystal field splitting observed for second and third row metal ions compared with their first row congener; Pd(II) and Pt(II) complexes are invariably square planar (but see Box 20.7).
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