Trans-effect geometrical

In 1928, Chemyaev used his trans effect to confirm the sqnare-planar configuration for platinum(ll) by preparing the three theoretically possible geometric isomers of a compound of type MABCD (15). The occurrence of three... 

Ligand substitution is one of the most characteristic reactions of coordination compounds. If in the early stages of the development of coordination chemistry a ligand substitution reaction served as a synthetic method, then later on, especially after Werner, such reactions were widely employed both to solve structural problems (viz., geometric isomerism), and to elucidate the nature of a trans effect. 

Infrared absorption peaks [Table], 234 Inhibitors, 39 Initiation, 34 Intermediates, 31, 41 Inorganic esters, 262, 272, 276 Inversion, 124, 139 Ion-dipole attraction, 23 Ionic bonds, 6 Isobutyl group, 146 Isolated bonds, 146 Isomerism, 2 alkyl halides, 118 cis-trans, 88 geometric, 88 optical, 70 Isomerization, 202 Isomers of butane, 50 of heptane, 66 of 2-hexene, 11 of pentane, 50 Isoniazide, 457 Isoprene rule, 181 Isopropyl group, 54 Isoquinoline, 458 Isotope effect, 130 lUPAC, 56... 

In due consideration of these results, we have decided to employ electrostatic forces to achieve a large reversible deformation of gels. The electrostatic force is expected to be a more effective driving force for the conformational changes of polymer chains than trans-cis geometrical isomerization of unsaturated linkages. The second part describes the photostimulated dilation of polymer gels. 

The synthesis of palladium polyamines (2) is a straightforward extension of our previous work involving the synthesis of the analogous platinum polyamines ( 6,7). The geometrical structure of the units about the palladium atom is believed to be as depicted in 2 where the like units are cis to one another. This is supported by the trans effect (8) discussed in the previous paper and by spectroscopic measurements cited in the following paragraphs. 

Cisplatin, cw-diamminedichloroplatinum(II), is an extremely potent antitumor agent. Show how it can be prepared in high purity to the exclusion of the trans geometric isomer by employing the kinetic trans effect. 

Many series of metal complexes which show a general additivity of substituent effects show small and systematic differences for geometric isomers. In Co(III) complexes with second row or Cl ligands the cobalt shielding increases and the linewidth decreases from the trans to the cis and from the met to the fac isomer, with decrease in the local electric field gradient. " " These correlations may vary, " depending on the trans effects of the ligands. 

Thus, such substitution reactions proceed both in the solid state and in solution in the manner predicted by the trans-effect. Peyrone s rule (Chapter 1) sometimes appears to be violated since isolated products may have undergone geometric isomerization subsequent to the initial thermal transformation. 

The cis-trans isomerization of square-planar transition metal complexes, in particular those of platinum(II) and palladium(II), can be understood by comparison of the trans-effects of the individual ligands (Chapter 1). This approach predicts the favored direction of isomerization for a large group of complexes with ligands of different types. In this Section examples of geometric isomerization which are not adequately explained by the trans-effect are considered and it is shown that solid state effects can influence the outcome of such reactions. 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

The molecular orbital description of the bonding in NO is similar to that in N2 or CO (p. 927) but with an extra electron in one of the tt antibonding orbitals. This effectively reduces the bond order from 3 to 2.5 and accounts for the fact that the interatomic N 0 distance (115 pm) is intermediate between that in the triple-bonded NO+ (106 pm) and values typical of double-bonded NO species ( 120 pm). It also interprets the very low ionization energy of the molecule (9.25 eV, compared with 15.6 eV for N2, 14.0 eV for CO, and 12.1 eV for O2). Similarly, the notable reluctance of NO to dimerize can be related both to the geometrical distribution of the unpaired electron over the entire molecule and to the fact that dimerization to 0=N—N=0 leaves the total bond order unchanged (2 x 2.5 = 5). When NO condenses to a liquid, partial dimerization occurs, the cis-form being more stable than the trans-. The pure liquid is colourless, not blue as sometimes stated blue samples owe their colour to traces of the intensely coloured N2O3.6O ) Crystalline nitric oxide is also colourless (not blue) when pure, ° and X-ray diffraction data are best interpreted in terms of weak association into... 

Where unidentate ligands are present, the ability to effect the resolution of an octahedral complex (i.e. to separate 2 optical isomers) is proof that the 2 ligands are cis to each other. Resolution of [PtCl2(en)2] therefore shows it to be cis while of the 2 known geometrical isomers of [CrCl2en(NH3)2] the one which can be resolved must have the cis-cis structure since the trans form would give a superimposable, and therefore identical, mirror image ... 

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