Distortion isomers

A number of Mo complexes of the type [MoOC12(PR3)3] have also been isolated in both blue and green forms.105 X-Ray structure determinations have shown that both forms possess the meridional stereochemical arrangement, with the oxo and the Cl- ligand irons to each other.106,107 The green form has a shorter Mo=0 distance (1.68 A) compared to the blue (1.80 A) which is unusually long. No satisfactory explanation for the existence of distortion isomers in the crystalline state has been forthcoming. 

Table 68 X-ray Crystallographic Data for Some Cation Distortion Isomers"...

Figure 81 The correlation of the structure and electronic energy levels of series of anion and cation distortion isomers...

A-type. Distortion isomers in which complexes of the same empirical formula can exist in the radically different stereochemistries, such as in a- and /J-Cu(NH3)2Br2 [19]68 and [20]69, which involve the compressed and elongated octahedral [CuN2Br2Br2] chromophore. In this system the /3-form may be converted to the a-form by application of pressure.70 The occurrence of this type of isomerism is much less common than the B-type. 

D) cation distortion isomers [Cu(dien) (bipyam)]X2 and [Cu(bipy)2Cl]X... 

Examples of anion distortion isomers with different geometries have already been observed in the octahedral and compressed octahedral CuN6 chromophores of K2Pb[Cu(N02)6] [1] and Rb2Pb[Cu(N02)6] [4] and in the compressed tetrahedral or... 

The real significance of anion and cation distortion isomers is that for a given chromophone, small changes in geometry may be observed, which can then be correlated... 

The above examples demonstrate that it is not possible to predict a precise stereochemistry for a copper(II) complex in view of the existence of distortion isomers of Type A and B and of cation or anion distortion isomers. Each will have a definite stereochemistry, but a range of possible distortions (within certain limits) can arise due to the Plasticity Effect of the copper(II) ion and its variable eccentricity. This means that crystallographic data will be essential in determining the precise stereochemistry of the copper(II) ion in these distorted geometries. It also opens up the possibility that through... 

Key words molecular mechanics, DFT, ligand-field theory, oxygen activation, Jahn-Teller distortion, distortional isomer... 

A molecular-mechanics scan of the potential-energy surface of [M(L )]"+ indicates that the two distortional isomers (M-N7oM-N3) are nearly degenerate for cobalt(II) (see Figure 18-5) this effect emerges also from experimental structural studies [25]. The lower-energy, nearly flat curve in Figure 18-5 indicates that the... 

S.E. Loehr. T.M. Eneraark. J.H. Studies of distortional isomers—Spectroscopic evidence that green cis,mer-dichlorotris(dimeth lphenylphosphine)-oxomolybdenum- 26. 

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. 

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