Square-planar, 16-electron complexes

Square planar complexes are also well authenticated if not particularly numerous and include [Co(phthalocyanine)] and [Co(CN)4] as well as [Co(salen)] and complexes with other Schiff bases. These are invariably low-spin with magnetic moments at room temperature in the range 2.1-2.9 BM, indicating 1 unpaired electron. They are primarily of interest because... 

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). 

Suggest the form that the orbital energy-level diagram would take for a square planar complex with the ligands in the xy plane, and discuss how the building-up principle applies. Hint The d -orbital has more electron density in the xy plane than the dzx- or d -orbitals but less than the dXJ,-orbital. 

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. 

Each step includes elementary acts that require different properties of the metal, for example, sufficiently low ionization potential to favor oxidative addition, sufficiently weak metal-carbon bonds, tendency to form square-planar complexes and to reach pentacoordination to allow insertion, a sufficiently high electron affinity to allow reductive elimination, and so on. Some properties are conflicting and a compromise has to be reached. 

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. 

Square-planar stereochemistry is mostly confined to the d8 transition metal ions. The most investigated solvent exchange reactions are those on Pd2+ and Pt2+ metal centers and the mechanistic picture is well established (Table XIV (194-203)). The vast majority of solvent exchange reactions on square-planar complexes undergo an a-activated mechanism. This is most probably a consequence of the coordinatively unsaturated four-coordinate 16 outer-shell electron complex achieving noble gas... 

Exceptions to the E.A.N. rule do occur, particularly with d8 metal ions, where many examples of square-planar, 16-electron complexes are known. In these complexes, the high-lying pz orbital is nonbonding and remains empty. This deviation from the rule is often said to be due to the large s-to-p promotion energies found for the free atoms. As the atomic number increases across a given transition-metal series, the... 

As seen, the square planar complex exhibits in MeCN solution a reversible one-electron oxidation (Eo1 = -0.12 V vs. Fc/Fc+ AEp = 70 mV, at 0.05 V s-1), which suggests the maintaining of the square planar CuN4 coordination. In fact, as illustrated in Figure 126, the Cu(III) derivative [Cu(Hmal)] is square planar.182... 

The square planar complex undergoes either two separate one-electron reductions = -1.03 V, A2sp = 66 mV, at0.2V/s 1 iTy2 = -1.40 V), or two one-electron oxidations (2sjy+=+0.52V, A2sp = 84 mV, at0.2V/s 1 +/2+ = +1-4V) (the second oxidation is not shown in Figure because its irreversible character masks the return peak of the first oxidation). It is easily conceivable that all the redox processes are ligand centred. 

The 16-electron square planar complex is converted into an octahedral 18-electron complex. In Figure 2.14 we have depicted the oxidative addition of methyl iodide to Vaska s complex (L=phosphine). Iodide ions accelerate the reaction and addition of an anion to the metal is the first step in that instance [10]. 

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... 

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

The Ni—N bond distances, N-N bite distances, and N-M-N bite angles of Ni(II) macrocyclic complexes depend on the coordination number of the metal ion and the type of macrocycle. These structural parameters influence the electronic spectra and the electrochemical data. In general, Ni—N bond distances of square-planar complexes are shorter than those of the octahedral complexes because of the absence of electrons in dx2 . Furthermore, as the Ni—N bond distance in-... 

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