Isomers ambidentate

It should be noted that, by convention, the ambidentate ligand is always written with its donor atom first, i.e. NO2 for the nitro, ONO for the nitrito, NCS for the A-thiocyanato and SCN for the 5 -thiocyanato complex. Differences in infrared spectra arising from the differences in bonding are often used to distinguish between such isomers. 

DMSO is an ambidentate ligand, capable of coordinating via either S or O. The m-isomers have three S-bound and one O-bound ligand while in the Ira/w-isomers all are S-bonded IR spectra show the presence of both S-and O-bound DMSO in the m-isomers with absorption owing to i (S-O) around 1100 cm-1 (S-bonded) and 930 cm-1 (O-bonded), while the trans-isomers only have i (S-O) around 1100 cm-1 (Figure 1.38). 

Some interesting cases of isomerism in bridged complexes do arise. The thiocyanate bridged complex shown in Figure 3.41 is a good example of the ambidentate behaviour of the thiocyanate (confirmed by X-ray) while in the complexes [Pt(PR3)(SR)(SR )]2 the choice of isomer is determined by the order in which the thiolate groups are introduced (Figure 3.42). 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

Many studies have been carried out on this reaction. Some of the unusual features of this reaction will be described in Chapter 20. Note that in writing the formulas for linkage isomers, it is customary to write the ligand with the atom that functions as the electron pair donor closest to the metal ion. Special consideration will be given in Chapter 20 to the behavior of cyanide complexes because CN-is also an ambidentate ligand. 

Sketch structures for all of the isomers of [Co(en)2ClNCS]+ assuming that SCN can be ambidentate. 

By taking advantage of the observation that the other ligands in a metal complex may alter the type of metal-atom bonding to an ambidentate ligand, it was possible to prepare thiocyanato (M—SCN) and isothiocyanato (M—NCS) linkage isomers. The syntheses of two such palladium (II) complexes are described below. 

The thermal isomerization of the purple S-bound fraM-[Co(NH3)4( NH3)SCN](N CS)2 complex (20) to the thermodynamically stable red N-bound isomer has been studied in the solid state (see Ambidentate Ligand), and the results are consistent with backside attack by ionic lattice... 

Linkage isomerism is but a special case of ambidentate behavior in ligands. The cyanide ion provides good examples of such behavior. In discrete complexes it almost always bonds through the carbon atom because of the. stronger tt bonding in that mode. It has also been reported to form a few linkage isomers such as cjj-[Co(lrien)(CNK] and c/4 4Co(trien)(NC)j]. ... 

The isomers are obtained by treating di-M-chlorobis[chlorotripropyl-phosphineplatinum(II)] with two equivalents of potassium thiocyanate— i.e., in cold acetone the a isomer forms in boiling acetone, the 13 isomer. There do not appear to be any examples of bridged linkage isomers where the ambidentate ligand is not involved in the bridge. 

The low quantum yields plus the interference of photoredox limits the synthetic applications of photochemistry among the Co(lll) amines. However, several applications involve linkage isomerization of ambidentate ligands, e.g., the photolysis of nitro complexes leads to nitrito isomers, presumably via a charge-transfer excitation. In H O both photoreduction to Co and linkage isomerization are noted ... 

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