Distortional isomerism

In their 1971 paper Chatt, Manojlovic-Muir and Muir proposed the term distortional isomerism to describe two forms of [Mo(0)Cl2(PMe2Ph)3], one blue 

A Journey in Metal-Ligand Multiple Bond Chemistry 

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Probably the most appropriate example of distortional isomerism is that observed with copper(I) complexes of the tetradentate ligands 14 and 35. Shown in Fig. 12(a) and b) are plots of the X-ray crystal structures of two forms of [Cu(14)(NCCH3)]" " [the five-coordinate form b) is red, the four-coordinate... 

This account would not be complete without mention of distortional isomerism, a term Joe Chatt was to coin in describing two isomeric forms of the six-coordinate compounds, [Mo(0)Cl2(PMe2Ph)3] (35, Figure 12) in a short communication published in Chem. Commun. in 1971, but which later was to spark an intense debate into the reality or otherwise of the phenomenon of bond stretch isomerism. We followed the debate with some interest, since we had inadvertently stumbled across seven-coordinate oxo and sulfido compounds of niobium of the type [Nb(E)Cl3(PMe3)3] (E = O or S 36) which seemed to show a closely related effect. 

Figure 18-3. Three types of isomerism with bispidine complexes (a) The variation of the metal ion site in the bispidine cavity, [25] (b) Jahn-Teller isomerism, [22] and (c) distortional isomerism of the copper(l) complexes [39]...

The presence of two distortional isomers was suggested for a number of metal oxo species, such as MoOCl2(PR3)3 (11.36). The blue and green isomers of this series was found to have different M=0 bond lengths. Parkin has found that MoCl3(PR3)3 (11.37) can cocrystallize with 11.36 in such a way as to cause an apparent lengthening of the crystallographically determined M—O distance, and so distortional isomerism may not be real. This is an illustration of how easy it is to miss alternative interpretations of the data. 

The flurry of recent interest in isomerism arose because of the suggested existence of an isomerism related to conformation isomerism, a isomerism which is variously called distortional isomerism or bond-stretch isomerism. The suggestion was made that a bond length in a given complex could have either of two very different values. This could arise, for instance, if there were two different bonding interactions, each leading to stability at a different internuclear distance. What was the ground state at one distance... 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

HOMO of transition state for cis-trans isomerization in 2-butene shows distortion of the 7t bond. 

This compound has an octahedral coordination sphere slightly distorted towards a trigonal prism. Oxidation by air leads to Ru2(S2CNEt2)5 which exists in two isomeric forms (Figure 1.52) [138]. 

As the number of carbon atoms increases, the number of possible isomers becomes larger. Whereas there are only two isomeric butanes, there are three isomeric pentanes. With hve carbons, in addition to the open-chain compounds shown, a stable ring compound known as cyclopentane is also possible. Five- and six-membered carbon rings are very stable because the bonds between carbon atoms in these size rings are close to the 109° angle preferred by carbon. Three- and four-membered hydrocarbon rings are also known, but they are less stable because of the required distortion of the bond angle. 

Previous studies have concluded that 4-, 5-, and 6-coordinate W species are present on AI2O3 and Ti02 supports [17,30] depending on surface W density and on hydration state. The present study has detected W03-like distorted octahedral domains at all surface densities and irrespective of hydration on Zr02. These species catalyze alkane isomerization reactions with much higher turnover rate and selectivity than dispersed WOx moieties on alumina or titania. 

These results are consistent with active sites consisting of highly distorted octahedral WOx clusters on Zr02. Acid sites formed by these octahedral WO surface species are more effective isomerization sites than previously reported tetrahedral WOx species on AI2O3 [17], possibly because of the ability of WOx clusters to form metastable proton-containing complexes during catalytic isomerization reactions. 

Density functional theory (B3LYP) has been used to investigate the mechanism of the cis-trans isomerization in Ir(H)2(CO)(C6H3 CH2P(H2) 2).300 The preferred mechanism involves two consecutive trigonal twists in which the complex passes through a distorted octahedral intermediate, as shown in reaction Scheme 19. 

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