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 d_z2 orbital is greatly stabilized; the energies of the d_yz and d_xz 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 d_xy and d_x²_-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 Δ_oct. 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).

مواضيع ذات صلة

ايجاد الصيغة التركيبية للمعقدات

معقدات الخارصين

معقدات النحاس

كيمياء ومعقدات بعض عناصر السلسلة الانتقالية الاولى

تعريف المركبات المعقدة

Crystal Field Stabilization Energies

Colors of Transition-Metal Complexes

The Nature of the Ligands

Principal Quantum Number of the Metal

Charge on the Metal Ion

Factors That Affect the Magnitude of Δo

Electronic Structures of Metal Complexes