Skew lines

In this case, the reference axis is drawn through the metal ion in a fashion perpendicular to the chelate ring. The skew line defining the helical sense (L for left-hand, and D for right-hand) is the bond from the chelate ring to the rest of the molecule. The L and D complexes are diastereomeric and can be separated from each other for direct stereochemical studies of their individual binding and inhibitory properties. 

Skew-fax convention for octahedral complexes. The second is the skew line conven-uon and applies to octahedral complexes. Tns(dideniate) complexes constitute a general family of structures for which a useful unambiguous convention has been developed based on the orientation of skew lines which define a helix. Examples I and 2 represent the delta and lambda forms of a complex, such as [Co NH2CHjCH,NH,)]3 i ... 

If Fig. 1 confront the observed parameters to the theory for p = 2. We can conclude that (1) all observations are consistent with the wind model (2) WR stars with radii larger than 10 Rg must exist the skew lines in Fig.l reflect the uncertainties on the radius if we continue the lines down to... 

Bridged binuclear, trinuclear and tetranuclear chelated octahedral structures were examined by Schaffer,17 who used the skew line helical definition for the chirality symbols A and A. The configurational isomers for a tetrakisbidentate, edge-fused-bis-octahedral structure are AA, AA, AA, and AA. 

Figure 7 Specification pf helicity in terms of skew lines representing axis and tangent...

Since these are linearly Independent In H°(X,ox(2))4, they span H°(X,1U Hence dg0 Is an Isomorphism and the claim follows from proposition (8.2). A similar argument proves that the family py Is effectively parametrized at every point of U. Therefore g maps to an open set of an irreducible component of H1lb3o 2 Applying (8.2) we see that Hl)b32t 2 is smooth at the closed points which parametrize pairs of skew lines. 

FIGURE 5.6 Definition of the skew-line convention used to define the absolute configuration of tris- or bis(chelate) complexes and conformation of diamine chelates. 

Upon complexation with a lanthanide ion, these complexes may form square antiprism or twisted square antiprism (TS APR) structures with a vacant coordination site in the cap position, which is assumed to be occupied by a solvent molecule. Just as in the chelated complexes described previously, two distinct types of chiral stereochemistry are present. In analogy with OC-6 species, the sense of rotation of the pendant arms is denoted as A or A depending upon if the arms rotate clockwise (A) or counterclockwise (A) as one proceeds down the direction of the C4 axis. There is also chirality (or helicity) associated with the nonplanar 12-membered ring. If one looks along the skew-line connecting the coordinated nitrogens, the carbon atoms... 

Fig. 5 Definition of skew-line chirality descriptors with rotational right- (8/ A) and left-(X/A) handed orientations...

The chiral identity of a molecule is included in the nomenclature of inorganic compounds, and today s comprehensive system is based upon suggestions made in 1990 in IUPAC s Recommendations on Nomenclature of Inorganic Chemistry [84], and ACS s Inorganic Chemical Nomenclature [90]. The basis for the usage of stereochemical descriptors was laid by Brown [91,92], from which three types of chiral descriptor conventions were developed (i) Steering-wheel-convention [93], (ii) Skew-lines convention [94] and (iii) Oriented-skew-lines convention [95]. 

Skew-lines are two non-intersecting lines that are neither orthogonal nor parallel to each other with a principal axis intersecting both skew lines orthogonally... 

The oriented skew-lines convention describes orthogonal skew lines that are not defined with the classical system. By analogy, the vector descriptors are chosen to be A/yl, respectively... 

It is equivalent to describe the symmetry class of the tetrahedron as 3/2-m or 3/4. The skew line relating two axes means that they are not orthogonal. The symbol 3/2-m denotes a threefold axis, and a twofold axis which are not perpendicular and a symmetry plane which includes these axes. These three symmetry elements are indicated in Figure 2-50. The symmetry class 3/2-m is equivalent to a combination of a threefold axis and a fourfold mirror-rotation axis. In both cases the threefold axes connect one of the vertices of the tetrahedron with the midpoint of the opposite face. The fourfold mirror-rotation axes coincide with the twofold axes. The presence of the fourfold mirror-rotation axis is easily seen if the tetrahedron is rotated by a quarter of rotation about a twofold axis and is then reflected by a symmetry plane perpendicular to this axis. The symmetry operations chosen as basic will then generate the remaining symmetry elements. Thus, the two descriptions are equivalent. 

The R/S convention for tetrahedral centres The R/S convention for trigonal pyramidal centres The C/A convention for other polyhedral centres The C/A convention for trigonal bipyramidal centres The C/A convention for square pyramidal centres The C/A convention for see-saw centres The C/A convention for octahedral centres The C/A convention for trigonal prismatic centres The C/A convention for other bipyramidal centres The skew-lines convention... 

Application of the skew-lines convention to tris(bidentate) octahedral complexes... 

The second is based on the geometry of the molecule and makes use of the skew-lines convention it is usually applied only to octahedral complexes. The two enantiomers are identified by the symbols A and A in this system. The C/A nomenclature is not required for those chelate complexes where the skew-lines convention is completely unambiguous (see Sections IR-9.3.4.11 to 9.3.4.14). 

The absolute configurations of some octahedral complexes can be described using either the skew-line reference system (Section IR-9.3.4.11) or the C/A system. The first is used more commonly, but the C/A system is more general and may be used for most complexes. The skew-line reference system is only applicable to tris(bidentate), bis(bidentate) and closely related systems. 

Tris(bidentate) complexes constitute a general family for which a useful, unambiguous convention has been developed based on the orientation of skew lines which define a helix. 

Figure IR-9.1. Two skew lines AA and BB which are not orthogonal define a helical system. In the Figure, AA is taken as the axis of a cylinder whose radius is determined by the common normal NN of the two skew-lines. The line BB is a tangent to the above cylinder at its crossing point with NN and defines a helix upon this cylinder. Cases (a) and (b) illustrate a right- and left-handed helix, respectively.

Figure IR-9.2. The figure shows pairs of non-orthogonal skew-lines in projection upon a plane parallel to both lines. The full line BB is above the plane of the paper, the dotted line AA is below this plane. Case (a) corresponds to (a) of Figure IR-9.1 and defines a right-handed helix. Case (b) corresponds to (b) of Figure IR-9.1 and defines a left-handed helix.

---