Ambidentate isomerism

Ambidentate ligands possess more than 1 donor atom and can coordinate through either one or the other. This leads to the possibility of linkage isomerism (p. 920). The commonest examples are the ions NO2 (p. 463) and SCN (p. 325). Such ligands can also coordinate via both donor sites simultaneously, thereby acting as bridging ligands. 

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... 

The sharp contrast in the chemical nature of Os(II) and Os(III) often results in a linkage isomerization of an ambidentate ligand accompanying such a redox change. To date, such isomerizations have been reported solely for the pentaammine moiety. Typically, through electrochemical measurements, a cyclic process is observed the oxidation of a species Os(II)-AB to Os(III)-AB is proceeded by an intramolecular... 

Particular interest attaches to the thiocyanate complexes since the mode of coordination of this ambidentate ligand is markedly influenced by the other ligands present 15 isomerization can be brought about in a number of cases by heating the solid complex or by dissolution in an appropriate solvent. The type of coordination found in a particular complex may in general only be rationalized by a consideration of both electronic and steric factors. As an example of the latter, frans-[Pd(NCS)2(PPh3)2] contains N-bonded, linear NCS groups, but in the... 

Return now to the questions surrounding the actual sequence of events leading to substitution following population of the reactive state. As in thermal substitution mechanisms it is appropriate to determine whether a dissociative or an associative mechanism obtains. Certainly, this point is the one most often clarified, but other aspects also deserve some scrutiny. These include the possibility of acid-base equilibria in the excited state, isomerization of potentially ambidentate ligands, the extent to which the extruded ligand is electronically or vibrationally excited, the degree of molecular distortion upon population of the reactive state and the possibility of competing chemical processes which may be influenced by the environment or by structural modifications of the molecule. 

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 other ligands present in the complex can affect the isomerism in two ways they can modify the class behavior of the metal, and they can impose steric restrictions on the coordination site of the ambidentate ligand. 

Since an ambidentate ligand possesses at least two groups capable of coordinating to metal ions, polynuclear complexes may also exhibit linkage isomerism. The selenocyanate complex of cadmium , [(C4H9)4N]2 [Cd2(CNSe)6], contains the dimeric anion on the next page,... 

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 ... 

Another classical example is nitrite ion, which offers N or O atoms as donors. This example has been deeply studied, and the way it behaves is fairly well understood. The O-bound isomer converts (isomerizes) to the thermodynamically stable N-bound isomer, sometimes even in the solid state, by an intramolecular process (without the ligand departing the coordination sphere) in inert complexes. Another feature of ambidentate ligands is that they can display a tendency to bridge between two metal ions, with each of the two different donor atoms attached to one of two metal ions. 

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