Square planar complexes reactivity

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. 

Reagent Reactivity. One of the most interesting aspects of substitution reactions of square planar complexes is that the reaction rates depend on the nature of the reagent. This permits a thorough investigation of the factors responsible for reagent reactivity towards these substrates. Note that this has not been possible for the reactions of most six-coordinated metal complexes, since their rates do not depend on the reagent. 

Ligand substitution reactions of square-planar complexes most often occur by associative reaction sequences. An example of square-planar organometallic complexes that will illustrate this reactivity is trani -Ir(Cl)(CO)(PPh3)2 (frequently referred to as Vaska s Complex), see Vaska s Complex). This complex undergoes rapid ligand substitution with CO, PR3, and... 

In the other two mechanisms (homolytic splitting and insertion) hydride formation is accompanied by formal oxidation of the metal, and reactivity is closely linked to the susceptibility of the latter to oxidation. Thus, the high reactivity of Co(CN).-, toward Ho, compared with that of Co(CNCH i)- reflects the tendency of CN" to stabilize preferentially the higher oxidation state, and CNCH the lower oxidation state, of cobalt. For square planar complexes the expected order of the tendency toward oxidation, and hence of reactivity toward Ho, is (subject to some modification by ligand variation),... 

The dimethylphenylphosphine complex frans- IrCl(CO)[P(CH3)2C6Hs]2 is. very useful in the study of the oxidative addition reaction. In addition to its high reactivity, the H nmr spectra of adducts of the complex are a convenient tool for obtaining stereochemical information about the addition of small molecules to square planar complexes.1... 

NO BF3, SOo, tetracyanoethylene, and O2 to form a series of well characterized products. The variations in reactivity toward such molecules as SOo, Oo, and BF that is observed among the many known square-planar complexes were attributed to the following factors (15) (a) the electron affinity of the covalent molecule, (b) the nucleophilicity of the metal in the complex, and (c) the ability of d orbitals on the metal to overlap effectively with suitable orbitals on the electrophile. Molecules such as Oo and SO2 are most appropriately viewed as tt acids of considerable electron affinity, although they do have some ability to act as (T donors, and also as Lewis (cr-bonding) acids. 

The period under discussion has seen intense interest in the recently discovered i7 -H2 complexes. The relevance of these to some isomerization reactions of square-planar complexes was reported in Volume 5 of this series, and is covered in another recent review. " More of these fluxional ds-dihydridoplatinum compounds have been reported, and the role of 17 -H2 derivatives in oxidative additions to d rhodium(I) and iridium(I) has been discussed. The increasing role of theoretical and bonding studies is reflected in four works relevant to 4-and 5-coordinate molecules. Electronic structure is related to chemical reactivity in the reactions of phosphine bases with d bis(l,l-dithiolato)platinum com-plexes. Huckel calculations on the reactions of bis(nitrogen donor) ligands with 16-electron platinum(II) complexes have been carried out, as has more work on symmetry selection rules for isomerization reactions, which includes pseudorotation of 5-coordinate complexes and square-planar to tetrahedral conversions of 4-coordinate molecules. ... 

Steric effects can also promote ligand-dissociation pathways in 16e square-planar. -complexes, as shown in Equation (4). Here, the steric strain of having three fairly bulky PPhs groups (cone angle = 145°) promotes the dissociation of one to form a far more reactive trigonal-planar 14e complex. 

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