Trigonal-planar ML3 complexes

A trigonal-planar ML3 complex can be formed by removing the two axial ligands from a TBP ML5 complex (2-67). The d-block orbitals may therefore be readily deduced from those established in the preceding section. 

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. 

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Figure 2.11. Derivation of the d-block orbitals for a trigonal-planar ML3 complex from those of an ML5 complex with a TBP geometry...

A trigonal-planar ML3 complex (6-27) belongs to the D31, point group and contains the following symmetry elements the molecular plane (cTh), three perpendicular planes (cr,), each of which contains one M—L bond, a Cs-axis and an Ss-axis which are co-linear and perpendicular to the CTh plane, and three C2 axes, each of which is co-linear with one of the bonds. Each type of symmetry element is illustrated in 6-28. 

Table 6.21. Character table for the D31, point group and the characters of the reducible representation F, for a trigonal-planar ML3 complex...

Figure 6.8. Symmetry-adapted cr orbitals for j trigonal-planar ML3 complex.

The separation of Pa into P (eq) and P (ax) leads to a considerable simplification of the determination of the symmetry-adapted orbitals. For the orbitals on the equatorial ligands, they are identical to those we have already determined for a trigonal-planar ML3 complex ( 6.6.3.2,... 

Trigonal-planar ML3 complexes with a 7t system on the ligands... 

In this last example, we shall analyse a trigonal-planar ML3 complex in which the ligands are considered to have two p orhitals perpendicular to the M—L bond as well as the a orbital that points towards the metal ( 6.6.3.). Thesep orbitals are writtenpn (6-32) andpx (6-33), depending on whether they are in the plane of the complex or perpendicular to it. By convention, each orbital p, is oriented in a clockwise sense ( p i corresponds to the opposite orientation). This system can act as a model for a complex in which the three ligands are double-face jt donors or tt acceptors (Chapter 3). 

Figure 6.11. 7T and7rj symmetry-adapted orbitals in a trigonal-planar ML3 complex. 

The analysis that follows is also applicable to ML3 complexes with a trigonal-planar geometry... 

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. 

Draw qualitative diagrams for the crystal field splittings in (a) a linear complex ion ML2, (b) a trigonal-planar complex ion ML3, and (c) a trigonal-bipyramidal complex ion ML5. 

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