Complexes of Ambidentate Ligands

An ambidentate ligand has the choice of using two different types of donor atom. Two that have been extensively studied in their bonding to platinum and palladium are sulphoxides and thiocyanate. 

The sterochemistry adopted between these complexes appears to be a balance between steric and electronic effects. 

Steric crowding increases as bigger alkyl groups are introduced so that [Pt[(Me2CHCH2CH2)2SO]4](CI04)2 has only O-bonded sulphoxides (IR). IR spectra can be used to distinguish between S- and O-bonded sulphoxide  

A synthetic route for the two picoline complexes relies on the fact that when the base was added to ci5 -Pt(DMSO)2Cl2, the (ranj-isomer is formed first. On standing, partial isomerization occurs to the m-form, which can 

Reaction of PdCl with KNCS leads successively to a precipitate of Pd(SCN)2 and the soluble salt K2Pd(SCN)4 (square planar, Pd-S 2.31-2.39 A). This reacts with Ph3As to form the S,S-bonded Pd(SCN)2(AsPh3)2 (kinetic product), which on heating gives the thermodynamically more stable N,N-bonded isomer  

CH2CI2 Cr cr OSMe2 Flgore 3.70 Synthesis of dialkylsulphoxide complexes. 

---

Thus it appears that the presence of two soft carbons on the palladium stabilizes the trans coordination of hard ligands and drives the selective coordination of ambidente ligands through their hardest atom. These results, as those described in the previous scheme, constitute other examples of the antisymbiotic effect which can be observed in soft palladium(II) complexes. 

The reactivity of ambidentate ligands toward organic centers has been reviewed (679). The review by Beck and Fehlhammer (56) includes a most useful section on methods used for the preparation of these complexes, and this aspect will not be emphasized here. The principles of formation of selenocyanate complexes have been reviewed (682). 

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. 

In complexes with ambidentate ligands, gold(I) almost invariably bonds to the softer end of the ligand, as is clearly expected from the above discussion. For example, the ligands NCO, S2O3" and SO bond through N or S rather than Similarly, SC normally bonds... 

The factors that affect linkage isomerization of ambidentate ligand complexes of transition metals have previously been reviewed by Baluhura and Lewis (72). We will summarize some of these factors. 

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... 

---