Square-planar ML4 complexes

In a square-planar ML4 complex, the metal is placed in the centre of a square whose corners are occupied by the four ligands. One can therefore consider, at least formally, that a square-planar complex is formed by removing two ligands from an octahedral complex, for example, those situated on the z-axis (2-36). To establish the structure of the d block, it is convenient to start from the results already obtained for octahedral complexes. 

The main difference between the d blocks of oaahedral and square-planar complexes concerns the number of nonbonding or weakly antibonding orbitals there are three in the former but four in the latter. 

---

Correlation between the orbital splitting patterns of octahedral MLg and square-planar ML4 complexes. 

With the energy level scheme shown in Fig. 8.9.2, we can easily derive two types of energy level diagrams those for halides, as shown in Fig. 8.9.3, and those for cyanides, as shown in Fig. 8.9.4. We will make use of these results to interpret some spectral data for square-planar ML4 complexes. 

Figure 3 Construction of the valence orbitals of a square-planar ML4 complex...

Figure 2.6. Derivation of the orbitals of the d block for a square-planar ML4 complex from those of an octahedral ML6 complex.

Figure 2.9. Correlation diagram linking the d-block orbitals of a square-planar ML4 complex and those of a tetrahedral ML4 complex, following the deformation shown in 2-55...

These are all 16-electron complexes (six for the bonds and ten in the d block). The lack of two electrons compared to the 18-electron rule arises because a nonbonding orbital on the metal remains empty. As in the case of square-planar ML4 complexes, this is the p orbital perpendicular to the molecular plane (2-69), which, although nonbonding, is too high in energy to be occupied. 

Give the shapes and relative energies of the d-block MO for a square-planar ML4 complex whose ligands are located on the bisectors of the X- and y-axes. 

We shall consider a square-planar ML4 complex with a d electronic configuration which possesses two alkyl ligands (R) in cis positions. The reductive elimination of R2 leads to the formation of a d ° [ML2] complex. An example is shown in 4-53, that involves the elimination of ethane from a complex that contains a chelating diphosphine ligand. 

Consider a square-planar ML4 complex in which each ligand possesses a n system made up of two p orbitals that are perpendicular to the M-L bonds. These orbitals are p and p, depending on whether they are perpendicular to the plane of the complex or in that plane. The orientation of these orbitals is shown below. 

This chapter and the next will introduce the use of d orbitals in transition metal complexes. First of all w e shall build up the orbitals of octahedral ML and square-planar ML4 complexes. These molecular levels will be used to develop the orbitals of fragments which is the topic of Chapters 17-20 so considerable time will be spent on this aspect. How the octahedral splitting pattern and geometry is modified by the numbers of electrons and the electronic nature of the ligands is also undertaken. 

This chapter is a continuation of the last in that the orbitals of our other molecular building block, a square planar ML4 complex, are developed. This is a little more complicated than the octahedral case however, we shall need to use the orbitals of both extensively in subsequent chapters. From the octahedral and square planar splitting patterns a generalized bonding model can be constructed for transition metal complexes. This, in turn, leads to the topic of electron counting. Finally, one distortion that takes a square planar molecule to a tetrahedron is discussed. 

The square planar system was different (Figure 16.1) in that oncp orbital at the metal, 16.4, found no symmetry match. There are four metal orbitals primarily of d character at moderate energies, and 16.4, which lies at an appreciably higher energy. It is unreasonable to expect that two electrons should be placed in 16.4 and therefore, stable square planar ML4 complexes have 16 valence electrons. A trigonal ML3 complex will also have one empty metal p orbital. 16.5, and a stable complex will thus be of the 16-electron type. Linear ML compounds have two nonbonding p AOs, 16.6, so here a 14-electron complex will be stable. 

In the first section of this chapter we built up the orbitals of a square planar ML4 complex. An alternative geometry would be a tetrahedral species. There are two basic ways to convert a square planar complex into a tetrahedral one. The 16.29 to 16.30 interconversion involves twisting one pair of cis ligands about an axis shown in 16.29. That will conserve D3 symmetry along all points that interconnect 16.29 with 16.30. In the other path the two tram L—M—-L angles are decreased, as shown in 1631, ultimately yielding the tetrahedron 16.32. This conserves Dad symmetry. The elements of this latter pathway have actually been developed in Section 15.4, so we shall briefly explore this distortion. A Walsh... 

In the first section of this chapter, v/e built up the orbitals of a square planar ML4 complex. An alternative geometry v/ould be a tetrahedral species. There are two basic ways to convert a square planar complex into a tetrahedral one. The 16.33 to 16.34 interconversion involves twisting one pair of cis ligands about an axis... 

---