In trans complexes

In the cis complexes, the torsion angle is usually smaller than that in trans complexes (Tables 16 and 17), leading to decreased antiferromagnetic coupling. In this case, the chains are helical and the compactness of this helix is a further factor that needs to be considered the more compact the helicoidal structure, the greater the antiferromagnetic coupling. 

Figure 7.1. The main steps involved in (trans) complex formation by a N4-macrocycle interacting with [M(OH)4]2. ...

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Pyridine 1-oxide, like pyridine, can act as a ligand in transition metal complexes, but unfortunately good stability constants are not known. However, Shupack and Orchin have found that the C===C stretching frequency of the ethylene ligand in trans-ethylene pyridine 1-oxide dichloroplatinum(II) varies linearly with the pA and hence with the C7-value (ct+ or a, respectively) of substituents in the pyridine oxide. The data for the above reaction series are included in Table V. 

Geometric isomerism can also occur in chelated octahedral complexes (Figure 15.7, p. 416). Notice that an ethylenediamine molecule, here and indeed in all complexes, can only bridge cis positions. It is not long enough to connect to trans positions. 

The wer-isomer does not react under these conditions. The properties of coordinated anions (e.g. Cl-) in these complexes depend on the trans-ligand in various ways. Thus in wer-IrCl3(PR3)3, the chloride trans to phosphine is more labile and thus readily removed by aqueous AgN03 (unlike the other two chlorides). Likewise i/(Ir—Cl) in the IR spectra of these complexes depends on the ligand trans to chloride. 

In the case of the trans-complex, only the two chloride ions are substituted, the trans-effect of ammonia being too low to give substitution with the result that white needle crystals of trans-[Pt(NH3)2(tu)2]Cl2 are formed [73],... 

Elimination reactions have been particularly studied in the case of dialkyls. They depend on the alkyl groups being cis trans-complexes have to isomerize before they can eliminate, and a complex with a trans-spanning diphosphine ligand is stable to 100°C (Figure 3.56). 

In many cases, it has been found that 7r-bonding ligands favour S-bonding. In a complex with both N- and S-bonded thiocyanate (Figure 3.74) the N-bonded group is trans to P while the sulphur-bonded thiocyanate is trans to the harder nitrogen ( anti-symbiosis ). 

In the 1920s, the Russian chemist Il ya Il ich Chemyaev systematized reactions of complexes of several metals, particularly platinum(II and IV), by noting that a ligand bound to a metal ion influenced the ease of replacement of the group trans to it in the complex [139]. 

A unique pair of stereoisomeric dimolybdenum amidinate complexes has been prepared and structurally characterized. The reaction of Li[PhC(NSiMe3)2] with dimolybdenum tetraacetate afforded trans- and ds-Mo2(02CMe)2[PhC (NSiMe3)2]2- While the acetates coordinate to the M02 core via a bridging mode in both compounds, the benzamidinates are bridging in the trans complex and... 

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

The magnitude of J(Pt—P) for some phosphine-substituted SiPt complexes depends on the covalency of the Pt—P bond (136). Such a bond is relatively ionic in a system where it lies trans to a ligand of high trans influence and J(Pt—P) is then small. The coupling in cis complexes decreased in the order Cl > C > Si and since a comparable situation has been observed for methyl-Pt compounds it is suggested that the effect is one of a- rather than tr-interaction. 

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

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