Complex square-planar

Square-planar complexes are extremely important in inorganie chemistry, and we will now discuss the bonding in these complexes from the perspective of ligand field theory. 

FIGURE 10.13 Coordinate System for Square-Planar Orbitals. 

TABLE 10.8 Representations and Orbital Symmetry for Square-Planar Complexes 

Derive the reducible representations for square-planar bonding, and show that their component irreducible representations are those in Table 10.8. 

The TT-bonding orbitals are also shown in Table 10.8. The orbital interacts with 

Square-planar Complexes.— In all investigated compounds the formal oxidation state of the metal atom is -t-1. 

A simple square-planar complex is PtCU . The coordinate system that we shall use to discuss the bonding in PtCh is pictured in Fig. 9-13. 

The metal valence orbitals suitable for a molecular orbitals are Sdx, 6s, 6px, and 6py. Of the two d ts valence orbitals, it is clear that interacts strongly with the four ligand r valence 

The 5 4s, 5d, and 5dxi, orbitals are involved in ir bonding with the ligands. The 5dx orbital interacts with ir valence orbitals on all four 

We can now construct an approximate energy-level diagram for PtCl( . We shall not attempt to pinpoint all the levels, but instead 

Since Pt + is and since the four chlorides furnish eight r and sixteen w electrons, the ground state of PtCh is 

Octahedral crystal field Square planar crystal field 

Any survey of square planar complexes generally deals almost exclusively with compounds of platinum (II). This discussion will do the same. It is not correct to think that such complexes are formed only by platinum (II) Pt(II), however, provides by far the largest number of examples and its compounds have been studied most extensively. Square planar complexes are largely found among the low-spin d system, examples of which are 

During the last half century much of the platinum(II) chemistry has been studied by Russian chemists. The name Chernyaev is almost synonymous with trans effect because it was he who called attention to this effect and used it in the synthesis of platinimi(II) complexes. The trans effect may be described as the influence which a ligand has on the ease of replacement of a group in the trans position. Thus, for a reaction of the type show n in Eq. (60), L is said to have a large trans effect if the reaction is fast but a small trans effect if the reaction is slow. 

Many qualitative and some quantitative observations now show that the trans effect of L decreases in the order  

Since the trans effect order is CD NH3, it can now be understood why the classical reactions of Reiset (1844) and Peyrone (1845) yielded trans-and cfs-[]Pt(NH3)2Cl2n, respectively. Thus Reiset started with the tetra-ammine, and in the final step the ammonia opposite chloride ion was more readily replaced yielding the trans isomer, Eq. (61). 

Instead Peyrone, starting with the tetrachloro complex, obtained the ds isomer because of the greater reactivity of the chlorides opposite each other than that opposite ammonia in the final step, Eq. (62). 

Kinetic parameters for substitution at four square-planar metal centers, viz. Pt(II), Pd(II), and Ni(II) (192), and also Au(III) (193), can be compared in terms of cyanide exchange at the respective [M(CN)4]2-complexes (Table VI). 

Kinetics and mechanisms of substitution at Pt(II) and Pd(II) have been reviewed and compared with respect to reactions of nitrogen bases such as imidazole, pyrazole, inosine, adenosine, and guanosine-5 -monophosphate with ammine, amine, pyridine carboxylate, and 

It has long been known that substitution at the anion of Zeise s salt, [Pt(CH=CH2)Cl3], is, thanks to the high trans effect of the coordinated ethene, very fast. Recent developments in low-temperature stopped-flow apparatus have now permitted the study of the kinetics of substitution at Zeise s and other [Pt(alkene)Cl3] anions in methanol solution. These substitutions obey the customary two-term rate law (i.e. with kohs = ki+ /s3[nucleophile]), with large negative AS values for the k2 term as expected for Sn2 processes (196). 

Kinetic studies of aquation of dinuclear [ traras-PtCl(NH3)2 2 (p-NH2(CH2)6NH2)]2+ established rate constants for the loss of the first and second chloride ligands (7.9 x 10-5 and 10.6 x 10-4s-1), and for the reverse anations (1.2 and 1.5M-1s-1). Reactivities here are very similar to those in analogous mononuclear systems [Pt(amine)3Cl]+ (204). A kinetic and equilibrium study of axial ligand substitution reactions 

Irradiation of [Pt(hfac)2] with an excess of ethene at 350 nm produced an X-ray-characterized five-coordinated intermediate containing both hfac ligands still bidentate (i.e. limiting A mechanism) (208). Quenching of photoexcited [Pt(terpy)Cl]+, and 4 -substituted terpy and NCS- analogues variously, by a range of Lewis bases, such as MeCN, DMSO, py, acetone, has been documented (209). 

The b2g and eg levels are nonbonding (no ligand a orbital matches their symmetry) and the difference between them and the antibonding aig level corresponds to A. 

FIGURE 10-13 4/, Molecular Orbitals (a orbitals only). (Adapted from T. A. Albright, J. K. Burdett, and M.-Y. Whangbo, Orbital Interactions in Chemistry, Wiley-Interscience, New York, 1985, p. 296. 1985, John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.) 

FIGURE 10-15 D4 , Molecular Orbitals, Including tt Orbitals. Interactions with metal d orbitals are indicated by solid lines, interactions with metal s and p orbitals by dashed lines, and nonbonding orbitals by dotted lines. 

9Symbols such as au alg, and b2g are additional symmetry labels useful in describing MOs with significant d character. See Footnote 7 in Chapter 3. 

Exceptions to the 18-Electron Rule (Although the six lowest energy MOs are shown as degenerate in the figure, this is an approximation that does not take into account interactions s and p orbitals ofthe metal.) 

Relative Energies of Metal d Orbitals for Complexes of Common Geometries 

Molecular Orbitals of Square Planar Complexes (Only o donor and it acceptor interactions are shown.) 

Sixteen-electron square planar complexes are most commonly found for ds metals, in particular those metals having formal oxidation states of 2+ (Ni2+, Pd2+, Pt2+) and 1+ (Rh+, Ir+). Some of these complexes have important catalytic behavior, as discussed in Chapter 9. 

Another approach to the study of the nature of transition states is to determine volumes of activation. A good system to study proves to be bromide substitution at rra/i5-[PtCl2(PEt3)2], because electrostriction effects in interchanging bromide and chloride should be small. For the 

The general rate law for substitution by a ligand L at platinum(n), as at other complexes, is 

Usually the kx term corresponds to solvent-assisted dissociation, the term to direct nucleophilic attack by L, and both paths make comparable contributions to the overall mechanism of substitution. There are, however, exceptions to this general picture. Thus, if the solvent is non-co- 

Of late, this has been a particularly rich field of study, with some recently recognized mechanisms being consolidated by further examples of their operation, and yet new routes being discovered. The fluxional nature of some five-coordinate intermediates and their possible role in isomerization mechanisms is particularly notable. 

Nuclear magnetic resonance spectroscopy enabled the two methyl groups of the acetylacetonate complex 15 to be distinguished in CDCI3 solution. 

Donors such as pyridine or PPhs catalyzed a rapid isomerization which resulted in nmr equivalence for the methyl groups. Reactions were first 

The sulfur-bonded complexes (16) (M = Ni, Pd, or Pt) convert from the trans form, in which they crystallize, to a 1 1 cis-trans mixture in solution (note that in this case the geometry refers to substituent positions above and below the coordination plane). In organic solvents, the rate 

Autocatalysis as an isomerization mechanism finds support in a number of recent publications. Tetrahydrofuran(thf) has no detectable catalytic effect on the isomerization of complex 15, but a slow geometry change in 

Kinetics and mechanisms of substitution at centres have been the subject of a general and extensive review.  

Sestili, C. Furlani, and G. Festuccia, Inorg. Chim. Acta, 1970, 4, 542. 

Platiniim(u).—General. The rate law common to almost all substitution reactions of square-planar complexes is 

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Similar MO treatments are possible for tetrahedral and square planar complexes but are increasingly complicated. 

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... 

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

Although less numerous than the square-planar complexes, tetrahedral complexes of nickel(II) al.so occur. The simplest of these are the blue (X = Cl, Br, I) ions,... 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

Square planar complexes, in which the four bonds are directed toward the comers of a square, are more common. Certain complexes of copper(II) and nickel(II) show this geometry it is characteristic of die complexes of Pd2+ and Pt2+, including Pt(NH3)42+. 

In the cis isomer, the two CH3 groups (or the two H atoms) are as close to one another as possible. In the trans isomer, the two identical groups are farther apart. The two forms exist because there is no free rotation about the carbon-to-carbon double bond. The situation is analogous to that with cis-trans isomers of square planar complexes (Chapter 15). In both cases, the difference in geometry is responsible for isomerism the atoms are bonded to each other in the same way. 

In addition to the tetrahedral and octahedral complexes mentioned above, there are two other types commonly found—the square planar and the linear. In the square planar complexes, the central atom has four near neighbors at the corners of a square. The coordination number is 4, the same number as in the tetrahedral complexes. An example of a square planar complex is the complex nickel cyanide anion, Ni(CN)4-2. 

Reaction of RhCl3 and sodium amalgam with triisopropylphosphine under a hydrogen atmosphere yields a distorted square planar complex RhH(PPr3)3 (Figure 2.66). 

Various bidentate ligands like dithiocarbamate afford monomeric square planar complexes specific examples are Pt(S2CNEt2)2 and Pt(Se2CNBu)2 (confirmed by X-ray). A similar structure is found for the dithiobenzoate Pd(S2CPh)2 one form of the dithioacetate is dimeric, a second form is a mixture of monomers and dimers. 

Square planar complexes of palladium(II) and platinum(II) readily undergo ligand substitution reactions. Those of palladium have been studied less but appear to behave similarly to platinum complexes, though around five orders of magnitude faster (ascribable to the relative weakness of the bonds to palladium). 

Square planar complexes of the type MABCD have three geometric isomers and in several cases all have been synthesized. Therefore, the isomers of [PtClBr(NH3)py] can be synthesized as shown in Figures 3.88-3.90. 

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 intimate mechanism of replacement in ds square planar complexes. L. Cattalini, Prog. Inorg. Chem., 1970,13, 263-327 (139). 

Solvent paths and dissociate intermediates in substitution reactions of square planar complexes. R. J. Mureinik, Coord. Chem. Rev., 1978, 25,1-30 (133). 

Splitting of d-orbitals in square planar complexes of copper(II), nickel(II) and cobalt(II). Y. Nishida andS. Kida, Coord. Chem. Rev., 1979, 27, 275-298 (94). 

Spot tests, 1, 552 Square antiprisms dodecahedra, cubes and, 1, 84 eight-coordinate compounds, 1,83 repulsion energy coefficients, 1, 33, 34 Square planar complexes, 1,191, 204 structure, 1, 37 Square pyramids five-coordinate compounds, 1,39 repulsion energy coefficients. 1,34 Squares... 

Molecular oxygen is transported throughout the body by attaching to the iron(ll) atom in the heme group of hemoglobin. The iron(ll) atom lies at the center of a square planar complex formed by nitrogen atoms. When the O, molecule attaches to the iron, the plane of the heme group becomes distorted. 

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. 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

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