Square planar complexes mechanisms

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 ... 

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

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

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... 

Mechanistic interpretation of activation volumes on square-planar complexes is complicated by the geometry. The sterically less crowded complexes may have loosely bound solvent molecules occupying the axial sites above and below the plane. Replacing them in the formation of a five-coordinate transition state or intermediate may result by compensation in relatively small volume effects. It is therefore difficult to distinguish between Ia and A mechanisms from the value of the activation volume. Nevertheless, the AV values are negative and together with the second-order rate laws observed, point to an a-activation for those solvent exchange reactions. 

Insertion of alkenes. Alkene insertions have also been widely studied and many insertion products have been isolated [31], Alkene insertions follow a migratory mechanism in the palladium and platinum square planar complexes with diphosphine ligands [18],... 

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

G. K. Anderson and R. J. Cross, Chem. Soc. Revs. 9, 185 (1980) for a comprehensive and incisive review of isomerization mechanisms of square-planar complexes. 

Although the combination of [Ir(COD)Cl]2 and LI was shown to catalyze the alkylation, amination, and etherification of allyiic esters to form the branched substitution product in high yield and enantioselectivity, the identity of the active catalyst in these reactions had not been identified. The combination of [Ir(COD) Cl]2 and LI forms the square-planar [Ir(COD)(Cl)Ll] (4) (Scheme 11) [45]. However, this complex does not react with allyiic carbonates to form an appreciable amount of an aUyl complex, and the absence of this reactivity suggested that the mechanism or identity of the active catalyst was more complex than that from simple addition of the allyiic ester to the square-planar complex containing a k -phosphoramidite ligand. 

The mechanism by which the d-d transitions gain intensity still remains to be determined. For octahedral and square planar complexes which have a center of symmetry, the transitions are partly inhibited. Slightly disorted octahedral and square planar metal complexes may have a fractional part of a d-d transition allowed, and this static distortion mechanism may be responsible for some intensity in many cases (I, 2). The fact that when Co(acac)3 catalyst was utilized in the oxidation, its absorption wavelength remained unchanged and its coefficient increased,... 

Cquare planar complexes are generally of the low-spin d8 type. This includes the four-coordinated complexes of Ni (II), Pd(II), Pt(II), Au(III), Rh(I) and Ir(I). The best known and most extensively studied are the compounds of Pt(II). The kinetics and mechanisms of substitution reactions of these systems have been investigated in considerable detail. Studies on complexes of the other metal ions are rather limited, but the results obtained suggest that their reaction mechanism is similar to that of the Pt(II) systems. This paper briefly surveys some of the available information, and presents the current view on the mechanism of substitution reactions of square planar complexes. 

Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path.

Briefly, as Prof. Basolo points out in his paper, and Prof. Basolo and Pearson pointed out nicely in a review on the trans effect, the situation in the ir-bonding theory is a stabilization of the trigonal bipyramidal intermediate because there are more orbitals in the trigonal plane available for 7r-bonding in the trigonal bipyramid than in the square planar complex. The same is also true in the --system. The trans effect is directional, and in my opinion, it has to be orbital or quantum mechanical. 

In the author s own laboratory the Cu(II)-catalyzed hydrolysis of the phosphate ester derived from 2-[4(5)-imidazolyl] phenol recently has been investigated146. The pertinent results are (a) the pre-equilibrium formation of a hydrolytically labile Cu(II)-substrate complex (1 1), (b) the occurrence of catalysis with the free-base form of the imidazolyl and phosphate moieties and (c) the extraordinary rate acceleration at pH 6 (104) relative to the uncatalyzed hydrolysis146. The latter recalls the unusual rate enhancement encountered above with five-membered cyclic phosphates and suggests a mechanism in which the metal ion, at the center of a square planar complex or a distorted tetrahedral complex, might induce strain in the P-O ester bonds (60). viz. 

Ideally, chemists hope to understand a number of reaction mechanisms well enough that predictions about a diverse assortment of complexes involving different metals, ligands, and reaction conditions can be made. A good example of a type of reaction for which this level of understanding has been achieved is substitution in four-coordinate square planar complexes. 

Mechanism of Nucleophilic Substitution in Square Planar Complexes... 

The Intimate Mechanism of Replacement in dH Square-Planar Complexes 1978... 

Although in the previous section the basic concepts related to substitution reactions were explained with reference to octahedral complexes, substitution reactions are also common in square planar complexes. Studies on these complexes have resulted in a great deal of knowledge of the mechanisms of these reactions, so a brief description of the topic is presented next. 

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