Square planar complexes, configuration

Like palladium(II) and platinum(II), gold(III) has the d8 electronic configuration and is, therefore, expected to form square planar complexes. The d-orbital sequence for complexes like AuC14 is dx2 yi dxy > dvz, dxz > dzi in practice in a complex, most of these will have some ligand character. 

The next most common coordination number is 4. Two shapes are typically found for this coordination number. In a tetrahedral complex, the four ligands are found at the vertices of a tetrahedron, as in the tetrachlorocobaltate(ll) ion, [CoCl4]2 (2). An alternative arrangement, most notably for atoms and ions with ds electron configurations such as Pt2+ and Au +, is for the ligands to lie at the corners of a square, giving a square planar complex (3). 

When using the eighteen electron rule, we need to remember that square-planar complexes of centers are associated with a 16 electron configuration in the valence shell. If each ligand in a square-planar complex of a metal ion is a two-electron donor, the 16 electron configuration is a natural consequence. The interconversion of 16-electron and 18-electron complexes is the basis for the mode of action of many organometallic catalysts. One of the key steps is the reaction of a 16 electron complex (which is coordinatively unsaturated) with a two electron donor substrate to give an 18-electron complex. 

Since the octahedral and tetrahedral configurations have the same number of unpaired electrons (that is, 2 unpaired electrons), we cannot use magnetic properties to determine whether the ammine complex of nickel(II) is Octahedral or tetrahedral. But we can determine if the complex is square planar, since the square planar complex is diamagnetic with zero unpaired electrons. 

Fio. 26. Energy level diagram of 3cP configuration (Cu" " ) in square planar complex or tetragonal crystal field (CF). Effect of bonding of the 3d-electrons with ligands is shown. 

In other instances, irradiation of the d-d transition leads to no observable reaction. Examples of this behavior are found for complexes having a variety of d electron configurations and coordinative geometries square planar Ni(II) (3d)3 in Ni(CN)42 124 and mww-Ni(gIy)2 124 square planar Pd(II) in Pd(CN)42-,124 and tra -Pd(gly)2 square planar Pt(II) in Pt(CN)42" (5d)3 124 octahedral Co(III) (3d)6 in a variety of complexes (cf. Sect. III-C and III-D). A striking example of this type of behavior is afforded by the nonreversible photoisomerization of cis-Pt(gly)2 (5d)8 to trans-Pt(g y)2 [reaction (2)].124 It has been proposed that irradiation of either of these square planar complexes leads to the same tetrahedral intermediate which decays exclusively to mwj-Pt(gly)2. This behavior may be contrasted with the reversible photoisomerization shown in reaction (3).3... 

The metal ions which form Square planar complexes with simple ligands have a -electronic configuration. The five degenerate d-orbitals of the uncomplexed metal ion split into four different levels in a square planar complex. Thus three orbital parameters are needed. to describe the ligand field d-sputtings. 

Almost all square planar complexes with simple ligands are diamagnetic and contain a metal ion with the -electronic configuration. Thus the ground state is lAig. The lowest energy excited states will be described separately for case 1 and case 2. 

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