The Square Planar ML4 Molecule

FIGURE 16.1. Orbilal interaction diagram for a square planar, /- 4/ 4 complex. 

While the resultant level splitting pattern looks complicated at first glance, it is quite simple to construct. Notice that there arc four levels, l)2ir + + 2 /,, . which 

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, we examine one distortion that takes a square planar molecule to a tetrahedron and two examples from the solid state. 

Thomas A. Aibright, Jeremy K. Burdett, and Myung-Hwan Whangbo. 

16 SQUARE PLANAR, TETRAHEDRAL ML4 COMPLEXES AND ELECTRON COUNTING 

Generalized orbital interaction diagram for a MLn complex where the ligands are arranged in a spherical manner around the transition metal. 

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Determine the MOs for the square planar molecule ML4 of D4h symmetry. [Hint. Set up right-handed axes a, n 1, % on each ligand.]... 

The bonding nature of the M—M contact between square-planar d8-ML4 molecules and, in particular, of the model dimer cA-[PtCl2 HC(OH)NH 2]2 has also been the object of a theoretical investigation. It was predicted that the dimer was stable towards dissociation into two monomers, with bonding energies in excess of 3 kcal/mol [114]. In the d8-ML4-dimer each met-... 

Examples of square-planar complexes include the d, 16-electron complexes shown in Figure 13-10. To understand why 16-electron square-planar complexes might be especially stable, it is necessary to examine the molecular orbitals of such a complex. An energy diagram for the molecular orbitals of a square-planar molecule of formula ML4 (L = ligand that can function as both a donor and v acceptor) is shown in Figure 13-11. " ... 

For a square planar complex ML4 where L is a a donor and n acceptor, sketch the following interactions (assume that the z axis is perpendicular to the plane of the molecule). 

Several other recent reviews contain material relevant to this section. An article by Blandamer and Burgess on the thermodynamics, kinetics, and mechanisms of solvation, solvolysis, and substitution in nonaqueous solvents contains a contribution on the controversial dissociative mechanism for isomerization of square-planar molecules. This is outlined in Section 5.5. A review of ligand substitution reactions at low-valency transition-metal centers contains sections on five-coordinate metal carbonyl complexes and on ML4 complexes (mainly tetrahedral configurations with L being a tertiary phosphine), as well as on acid- and base-catalyzed reactions. A review by Constable " surveying the reactions of nucleophiles with complexes of chelating heterocyclic imines contains a sizable section on square-planar palladium and platinum derivatives. Most discussion centers on [Pt(bipy)2] and [Pt(phen)2] (bipy = 2,2 -bipyridine phen = 1,10-phenanthroline). The metal center, ligand, or both are susceptible to nucleophilic attack and the mechanisms involved are critically assessed. 

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