Square planar fields

Octahedral crystal field Square planar crystal field... 

V S, C M Kelly and C R Landis 1991. SHAPES Empirical Force-Field - New Treatment of igular Potentials and Its Application to Square-Planar Transition-Metal Complexes. Journal of American Chemical Society 113 1-12. 

Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

Four-coordinate, planar iron(II)-dithiolate complexes also exhibit intermediate spin. The first example described was the tetraphenylarsonium salt of the square-planar bis(benzene-l,2-dithiolate)iron(II) dianion, (AsPh4)2[Fe(II)bdt2], which showed 5 = 0.44 mm s and AEq = 1.16 mm s at 4.2 K [157]. The electronic structure of a different salt was explored in depth by DFT calculations, magnetic susceptibility, MCD measurements, far-infra red spectroscopy and applied-field Mossbauer spectroscopy [158]. 

The square planar Ni11 complex (131) is formed upon reaction of MeCN with the dinuclear complex (132). A sequence of Ni-mediated hydrolytic transformations, MeCN —> acetamide and NH3 — acetamidine — TV-acetimidoylacetamidine, is proposed.488 While the A-acetimidoylacet-amidine ligands are isoelectronic with acac, they have a much stronger ligand field. 

FIGURE 17.9 Energies of orbitals in a square planar field produced by four ligands. 

On the basis of ligand field effects, would it be easier to form a trigonal bipyramid transition state from a square planar or a tetrahedral starting complex ... 

Although Chapter 25 does not address directly why some compounds with coordination 4 are tetrahedral and some are square planar, it is possible to surmise that the answer lies with (1) Crystal Field Theory and the energies of the d orbitals involved bonding and (2) how many unpaired electrons the metal complex has. 

Ammine is a strong field ligand. The ligand field diagrams follow, octahedral at left, tetrahedral in the center and square planar at right. 

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