Ligands rearrangements

An NMR study of steric and hyperconjugative barriers in benzyltrimethyl X derivatives (X = C, Si, Ge, Sn, Pb) has been reported/ The steric barrier to rotation about the C(sp )—C(sp ) bond for the compound X = C was shown 

Trimethylpalladium(IV) complexes are highly unstable but structures have been assigned to organo-palladium(IV) complexes with flexible bidentate ligands such as (py)2CHMe. Conformational and fluxional effects have been reported and compared with related Me3Pt(IV) systems. 

Five-membered ring pseudorotations are normally very facile and unable to be monitored by NMR. The hafnocene complex [(r -Cp)2Hf(l,2-Te2C6H4)] contains a five-membered HfTeC2 ring which adopts an envelope conformation, but this undergoes rapid reversal at room temperature/ However, in the spiro compounds steric and electronic factors significantly raise the pseudorota- 

Six-membered ring reversals normally involve chair-chair interconversions. However, the central ring of 9,10-dihydrosilaazaanthracenes and dibenzometal-lacyclohexa-2,5-dienes adopt boat conformations and boat-boat interconversions were examined by 2D-EXSY and variable-temperature ID The 

Two papers deal with the effect of co-ordinating cations on the case of rotation about carbon-nitrogen bonds in organic molecules. The effect of the variation of oxidation state of the cation on the barrier to rotation has been probed for a series of substituted dithiocarbamato (R2dtc) complexes of iron. In the iron(n) derivatives [Fe(R2dtc)a(phen)] and [Fe(R2dtc)2(bipy)], AG is ca. 8.6 kcal mol in [Fe (R2dtc)3] AG is ca. 12 kcal mol and in [Fe (Radtc)3]+ 

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On carbonylation in methylene chloride it undergoes the substitution of the diene ligands, rearrangement, and cis-trans isomerization to yield 64 (97JOM(530) 259). l,r-(l,2-Ethylene)-3,3 -imidazol-2,2 -diylidene and the dimer [(T -cod)Rh (ir-Cl)] form the dinuclear species 65. 

Os3(CO)i2, nine products were obtained. Three of these were identified as the substituted adducts Os3(CO)12(Ph3P)i2- Gc = 1, 2, or 3), but for others, the fission of C-H and C-P bonds had occurred to yield a variety of compounds in which there were radical variations in the structure and mode of bonding of the triphenylphosphine group. Figure 29 shows details of the structures of six of these complexes. As indicated above, significant ligand rearrangements have taken place. 

Incorporation of a third acetylene molecule takes place by CO replacement and without interference with the metallacyclopentadiene ring (170, 371, 379). In the cluster, then, the three acetylene ligands rearrange to a triacetylene ligand of unknown structure before the benzene is liberated (371, 379). 

B. Photorearrangement reactions may involve (i) geometrical isomerization, (ii) recemizatidn, (iii) Linkage isomerization, and (iv) ligand rearrangement. 

We have been able to demonstrate by synthesizing the unsymmetrical compound 87 and recording its temperature-dependent H NMR spectra that, in addition to the equilibrium between the enantiomers 86a and 86b, further ligand rearrangements need also to be considered. 

By plotting 5 S0/5J° vs. gj(gj — 1 )J(J + 1 )/S J° for all the lanthanides, the intercept gives / , and hence the ratio between pairs of pseudocontact shifts (Fig. 2.22). Within this approximation, the requirement for axial symmetry was found to hold for many systems, even with low symmetry, possibly as a result of ligand rearrangement which might be fast on the NMR time scale. 

There are attempts to use other iron nitrosyls, both monomeric complexes and clusters [85-90]. Photochemical liberation of NO follows three main reaction pathways (1) photooxidation-substitution, (2) photoreduction, and (3) ligand rearrangement or decomposition as a result of photoreaction. 

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