Trans complexation

Although analogous to the direct coupling reaction, the catalytic cycle for the carbonylative coupling reaction is distinguished by an insertion of carbon monoxide into the C-Pd bond of complex A (see A—>B, Scheme 31). The transmetalation step-then gives trans complex C which isomerizes to the cis complex D. The ketone product E is revealed after reductive elimination. 

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). 

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... 

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. 

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

Alkyls with groups that cannot / -eliminate (Me, CH2SiMe3) are more stable than those that can (e.g. ethyls). Trans-complexes that cannot eliminate by reductive coupling may -eliminate ... 

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

Trans- complex is obtained only with Cu11 which is coordinated to four oxygen atoms of two hfac ions and two nitrogen atoms of two TTF—CH=CH py ligands. Cu11 lies on inversion center and therefore the TTF—CH=CH py ligands are in trans- conformation. The copper ion adopts a Jahn-Teller distorted octahedral... 

Distorted trigonal bipyramidal, with R = n-Bu. 229, 230 NH- -Cl hydrogen bonds are formed, as attested by IR spectra and some of the foreshortened N—Hal distances. Other similar structures are known, such as R = Ph, which also yields an aW-trans complex with two molecules of l-methyl-2(3H)-imidazolinethione, that has octahedral configuration. 

The crystal structures of a number of s-trans (diene)metal complexes (3) have been determined10-14. The diene ligand in all s-trans complexes is distinctly non-planar the torsional angle between the two olefin groups is between 114° and 127°. In general, the terminal carbon to metal distance is greater than for the internal carbon to metal distance, and the Cl—C2/C3—C4 bonds are shorter than the C2—C3 bond. 

As seen, the two isomers result in structurally different metal complexes. In fact, in the octahedral trans complex, the two NH2 groups are positioned axially, whereas in the cis complex the two NH2 groups are positioned in close proximity imposing a trigonal prismatic geometry. 

In aqueous solution (pH 7) the trans-complex exhibits a reversible one-electron reduction at significantly negative potentials (.Eo = -0.81 V), whereas the cts-complex is reduced at less negative potential values (E° = -0.70 V).131 132... 

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