Meridian isomers

The facial isomer of the complex [RI1X3P3] has equivalent phosphorus atoms, so the spectrum is a doublet due to rhodium-phosphorus coupling, whereas the meridianal isomer has two types of phosphorus atoms, a pair mutually traws(b) and one irons to X(a) (1). 

More results for tn5-(phosphine) complexes, particularly for the meridianal isomers, would be a valuable aid to the interpretation of W-P coupling constants. We noted earlier (p. 368) that, for a given ligand, the couplings in these compounds may depend mainly on the trans ligand, and if this proves to be correct, the results of Table VIII suggest that in octahedral complexes this is unlikely to arise from the dominance of 7t-interactions. Until more results are available, it may be best to restrict interpretation to comparisons between complexes of the same microsymmetry (Section IV.C.8). 

The family of complexes MCI3B3, where M is Ru or Os, and B is a base such as tertiary phosphines and arsines, thio ethers, or pyridine has been studied in detail by Hudson and Kennedy 358). The complexes are mainly the meridianal isomer C ), but in one case, OsCl3(PBu2Ph)3, the g tensor indicates the other isomer, namely, facial C f). 

In a a 5-isomer, three Cl ions are on the triangular face of the octahedron and three NH3 molecules are on the opposite triangular face of the octahedron, hence it is called a facial isomer, abbreviated asjac-. While in the case of a trans-isomer, three CP ions are around the edge of the octahedron and three NH3 molecules are at the opposite edge of the octahedron, hence this is called a peripheral or meridianal isomer, abbreviated as mer-. 

A similar type of isomerism occurs for [Ma3b3] octahedral complexes since each trio of donor atoms can occupy either adjacent positions at the comers of an octahedral face (/hcial) or positions around the meridian of the octahedron (meridional). (Fig. 19.12.) Geometrical isomers differ in a variety of physical properties, amongst which dipole moment and visible/ultraviolet spectra are often diagnostically important. 

In this isomer, three of the same ligands lie on an equatorial circle about the complex. Such a circle is called a meridian, and this isomer is the meridional isomer, abbreviated mer. 

In the/flc isomer, the three chloride ions are located on the corners of one of the triangular faces of the octahedron. In the mer isomer, the three chloride ions are located around an edge (meridian) of the octahedron. The IUPAC system of nomenclature does not use this approach. A summary of the IUPAC procedures is presented in the book by Huheey, Keiter, and Keiter that is cited in the references listed at the end of this chapter. 

Structures I and II are identical. Hence, the facial (fac) geometrical isomer has no optical isomer. Structures III and IV are also identical. So, the meridianal (mer) geometrical isomer also has no optical isomer. The total number of isomers is 2 for [Cr(NH3)3Cl3]. 

The compound [Coptn2]Cl3 was also partly resolved into a dextrorotatory form. This was probably the fac-ABA cis B form—the only possible optically-active isomer. The others are mer-ABA and fac-ABA trans B where ptn = ABA, fac = facial or occupying one face of the octahedron, and mer = meridianal or positioned along the edge of the figure 45). Stability constants of ptn have been reported 37). 

The terms mer (meridional) and fac (facial) are used commonly to distinguish between stereoisomers of complexes of the form [Ma3b3]. In the mer isomer (Example 2) the two groups of three similar donors each lie on a meridian of the coordination octahedron, in planes that also contain the central atom. In the fac isomer (Example 3) the two groups of three similar donors each occupy coordination sites on the comers of a face of the coordination octahedron. 

In the other isomer, the three similar ligands lie in the same plane this is called the ner isomer (so called because the plane is analogous to a meridian on a globe). 

Figure 25-3 Ball-and-stick models of fbc-[Pt(NH3)3Cl3]+ left) and mer-[Pt(NH3)3Cl3]+ right). The three chloro ligands in the foc-isomer are all on one triangular face of an octahedron, and the three ammine ligands are all on the opposite triangular face. The mer isomer has the chloro ligands in one meridian (in a plane) and the three ammine ligands in a perpendicular meridian.

Fac (or cis) and mer (or trans) isomers of tri-aquatrichlorochromium(III). (o) The fac (or cis) isomer with the triangular face of chloride ligands outlined (dashed lines), (b) The mer (or trans) isomer with the chlorides along half the meridian outlined (dashed lines). Both geometric isomers possess an internal mirror plane (M) and are nonchiral. 

The meridian (mer) isomer, which has the three CO ligands in the same plane, is also shown in Figure 6.26 and gives a C2v structure. 

Figure 6.26 The facial (fac) isomer and meridian (mer) isomer of the general complex MLi(CO)-i. In each case the upper diagram shows a sketch in the normal orientation for octahedral complexes and the lower pictures use a view that should make the symmetry elements easier to see. The basis arrows along carbonyl bonds that are used in the vibrational analysis of carbonyl stretching modes are drawn slightly to the side of each ligand for clarity. Note that, for the C2V mer-isomer, the basis vectors bi and bi are symmetry related to each other, but not to b. ...

If a third CP is substituted for an NH3 in Figure 24-6(a), two possibilities exist. If this third substitution is at either the top or bottom of the structure, the result is that three CP ions appear on the same face of the octahedron. This is called a/flc (facial) isomer. If the third substitution is at either of the other two positions, the result is three CP ions around a perimeter or meridian of the octahedron. This is a mer (meridional) isomer. 

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