Ligand bulky

Significant quantities of the diphenoquinone are also produced if the ortho substituents are methoxy groups (36). Phenols with less than two ortho substituents produce branched and colored products from the reactions that occur at the open ortho sites. It is possible to minimize such side reactions in the case of o-cresol oxidation by using a bulky ligand on the copper catalyst to block the open ortho position (38). 

The stetic constraints imposed by the bulky ligands cause the propylene to bond almost entirely with a single orientation with respect to the growing polymer chain, CH2, which leads to the stereoregular product. 

In recent years this simple picture has been completely transformed and it is now recognized that the alkali metals have a rich and extremely varied coordination chemistry which frequently transcends even that of the transition metals. The efflorescence is due to several factors such as the emerging molecular chemistry of lithium in particular, the imaginative use of bulky ligands, the burgeoning numbers of metal amides, alkoxides, enolates and organometallic compounds, and the exploitation of multidentate... 

Some typical compounds are listed in Table 30.3. Coordination numbers below 6 are found only with very bulky ligands and even... 

As with other transition elements, the lanthanides can be induced to form complexes with exceptionally low coordination numbers by use of the very bulky ligand, N(SiMe3)2 ... 

As far as investigated77, most reactions of the allyllithium-sparteine complexes with electrophiles proceed antarafacially, either as SE2 or anti-SE2 reactions. As a working hypothesis it is assumed that the bulky ligand obliterates the Lewis acid properties of the lithium cation. 

Using more basic (electron-donating) ligands generally speeds up the reactions while bulky ligands slow it down. The use of tris(o-tolyl)phosphine is an extreme example of the latter as IrCl(CO)[P(o-tolyl)3]2 fails to add 02, H2 or S02 and only adds HCI slowly the reason is that methyls in an ortho-position tend to block the axial positions (Figure 2.77) [126b]. 

On reaction with 02, the P—Ir-P colinearity is maintained (174.5°), keeping these bulky ligands apart, but the Cl-Ir-C angle closes (to 97.4°) to accommodate the incoming 02 molecule. The geometry of the 02 adduct can be described as distorted octahedral, with Ir, Cl, C and the two O atoms essentially coplanar. As expected as the coordination number increases, there is a general increase in bond length (except for Ir—Cl). 

A number of tertiary phosphine complexes with bulky ligands (Figure 3.80) have modified square pyramidal structures, examples being M(I)3Br2, Pt(II)3Br2 and Pd(III)3Br2 (all X-ray) [136]. 

Very bulky ligands, of course, cause steric effects. The similarity of the Pt-P bond lengths in trans-Pt(PR3)2FI2 (R = Me, cy) suggest that the bonding is similar in these two compounds (Table 3.16 and Figure 3.95)... 

The phosphine and arsine complexes of gold(I) have been intensively studied since the early 1970s. The possibilities of coordination numbers between 2 and 4 have been explored, though the use of bulky ligands is less essential than with the isoelectronic M(PR3)2 (M = Pd, Pt) compounds and the coordination numbers depend on both steric and electronic factors [71]. 

The last two reactions are useful for esr studies involving free radicals. Until recently, the only trialkyltin radical that had been observed directly, in solution, by esr was MesSn- (295), but many more have now been reported (e.g., EtsSn-, PrsSn-, and BusSn ) (296). Bulky ligands [e g., (PhCMejCHjlaSn ] increase the persistence of the radicals, so that esr observation is easier (297), and tris(2,3,5-trimethylphenyl)tin and tris(2,3,5-triethylphenyl)tin radicals, at 180° and 100°, respectively, are in thermal equilibrium with the corresponding hexaaryldi-tins (298). 

Banditelli, G., Bandini, A.L., Bonati, F. and Goel, R.G. (1982) Some New Gold(l) complexes with bulky ligands. Gazzetta Chimica Italiana, 112, 539. 

This bulky ligand has two thiols to produce small Au nanoparticles. Figure 5 presents the TEM images of self-assembled monolayers of 1.5nm TCgBIP- and 1.6nm TCsBIP-protected Au nanoparticles at the air-water interface. 

Some Pt complexes bearing electron-donating phosphines can also catalyze the hydration of the C=C double bond of acrylonitrile or crotonitrile and yielded P-hy-droxypropionitrile or P-hydroxybutyronitrile respectively besides the corresponding amide (Eq. 6.38) [22, 75], Among the platinum phosphine complexes examined, [Pt(PEt3)3] (26a), carrying less bulky ligands, was the most effective for the hydration of the olefmic bond. The present catalyst system was ineffective for hydration of other olefins, however. 

We saw previously that a major factor in inhibiting the bimolecular termination reaction was the presence of sufficiently bulky ligands so that a monomeric dioxygen adduct could be isolated 135). A number of synthetic metal porphyrins 239) have been prepared recently which satisfy the above requirement, and bind molecular oxygen we shall now proceed to discuss these. 

---