Cis interactions

H-NMR studies of oligocarbene Ru(II) complexes indicate a substantial barrier to rotation about the metal-carbene carbon and nitrogen-R bonds. This restricted rotation is thought to arise as a consequence of intramolecular non-bonding cis interactions of the carbene nitrogen-R substituents, and not because of any significant double bond character in ruthenium-carbene carbon (76). 

In general, in square-planar and octahedral complexes 1JML depends on the trans-ligand stronger donors reduce the 1JML-value [10]. Similarly, ] i pM.iv.-val-ues are usually greater for trans-interactions than for cis-interactions [10], which is clearly an important tool for structural characterization (see Fig. 11.4). 

The interpretation of interaction parameters is far from stredghtforward. Calculations on model systems (2 c), and comparisons between parameters and force constants in real systems (4), show that the parameters contain several different terms. Thus in octahedral carbonyls, the cis interaction parameter, which is smaller than the trans parameter, originates mainly from the true cis interaction force constant. The trans interaction parameter, on the other hand, contains only a small direct contribution, while the main terms are indirect and involve MC,MC and MC.C O interaction constants. Calculations on substituted... 

Figure 4. Side and top views of the (16-pyridinium crown-4j cation showing the C-H -CI interactions with two guest chloride anions. ...

Ab initio calculations on the structures of ethylchlorocarbene and chloromethyl-chlorocarbene show that the equilibrium between cis and trans conformers is shifted moderately to the trans isomer for the ethyl derivative and strongly to the cis for the chloromethyl (due to a stabilizing carbene n — rearrangement transition states are product-like and the Z-isomers are stabilized by a through-space interaction of the pz orbitals localized on the terminal atoms. 

Figure 4.20 The environment of the gold atoms in Cs2Au Au "C16. Weak Au-CI interactions...

The position of equilibrium, as would be expected, lies strongly towards the pyranose form ([/]/[p] = 0.057), " the furanose form being disfavoured by the cis interactions of hydroxyalkyl chain on C4, OH C2 and pyrophosphate on Cl. A positional isotope exchange experiment, monitored by the isotope effect... 

All siglecs, with the exception of siglec-1, are "masked" by cis interactions with sialic acids on the same cell surface, thereby preventing cell-cell interactions. On cell activation, by an unknown mechanism, the siglecs seem to be "unmasked." Modulation of siglec binding can also be controlled by post-transla-tional modifications in their cytoplasmic domains. For example, protein kinase C phos-... 

Intramolecular heterolysis involves proton transfer to a cis ligand L (e. g. H or Cl) or to the counteranion of a cationic complex. This can occur via the intermediacy of a so-called cis-interaction, which essentially is a hydrogen-bonding like interaction of H2 with a cis ligand, such as a hydride, that has a partial negative charge (S-) [2, 5aj. 

A FIGURE 6-3 Schematic model for the generation of cell-cell adhesions. Lateral interactions between cell-adhesion molecules (CAMs) within the plasma membrane of a cell form dimers and larger oligomers. The parts of the molecules that participate in these cis interactions vary among the different... 

Brummendorf, T, and V. Lemmon. 2001. Immunoglobulin superfamily receptors cis-interactions, intracellular adapters and alternative splicing regulate adhesion. Curr. Opin. Cell Biol. 13 611-618. 

The story was not completely over, however, because the 1.49 A distance between H and Hbin 18 is short for a hydride (other dm are longer than 1.75 A), and Ha, Hb, and Hc are coplanar, both of which details were not reproduced by the elaborate calculations. Further calculations by Eisenstein rationalized the coplanarity as due to cis interactions (see Section 4.11), where the absence of an H-H bond does not affect the attraction between the hydrides. The reason that the H,-Hb bond is essentially broken (and not an H2 ligand) is the presence of sufficient (but unquantifiable) BD into nonbonding orbital different in energy than other backdonating orbitals that apparently spread the other hydrides further apart. 

Extended Huckel calculations demonstrate that this structure is the minimum. The system [Cp OsH2(PR3)(H2)] + is remarkable in that a complete structural change occurs to give two cis interactions (H---H-H--H) on changing R from Ph to Cy. dm increases from 1.01 to 1.31 A (PCy3 is a better donor), the H2 rotates 90°, and the H-H2 distances are 1.54(3) and 1.68(3) A,181 as in a putative H4... 

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