Palladium mixed-valence complexes

Another type of complex for which it is obvious thatQlarge structural changes occur on excitation (probably 0.2-0.3 A) along particular coordinates are the infinite-chain mixed-valence complexes of platinum and palladium (class II, or localized mixed-valence complexes) (11).. These Pti [/PtIV, PdIVPtIV or Pdi [/PdIV... 

Recent synthetic and spectroscopic work has concentrated on the study of structurally related linear-chain palladium complexes (7, 8), on the possibility of synthesising related nickel complexes, and on the study of mixed-metal mixed-valence complexes of the sort... 

It was speculated that the catalyhc achvity was a result of the reduction of one palladium(ll) center under the reaction conditions. This palladium center was stabilized as a mixed-valence complex, and entered the catalytic cycle via oxidative addihon of the aryl halide. It was also shown that heterotrimetallic complexes, bearing additional bridging Zn atoms, gave poor conversions and led to the formation of palladium black. A maximum TON of 490 was observed in the reaction of 4-bromoacetophenone with phenylacetylene at 0.2mol% catalyst loading in NEts at 90 °C. An important observahon here was that the addition of Cul did not improve the catalyhc activity rather, the use of this additive caused an overall decrease in the chemical yield. 

Other compounds of Pd + show a similar mixed valency behaviour-There is a striking similarity between the ionic radii of Pd + and Pd +, that has been proved by means of structure analysis of the chloro-complexes of two- and four-valent palladium (30). This is the reason, why the formulation Pd2+(Pd4+Fe) is not contradictory to the equivalence of the Pd sites found in the PdFs-structure (139). On the other hand it seems quite reasonable to assume a LiSbFe-type structure of compounds like Pd2+(PtFe), containing cations of different sizes. 

Mixed-valence linear-chain complexes of palladium display very similar features to those of platinum (97). Indeed, the great intensity of the t j progression in all these cases means that resonance Raman spectroscopy provides a sensitive and rapid means for detecting the formation of new linear-chain complexes of this sort. 

Nickel(II), palladium(II) and platinum(II) form planar complexes with 1,2-diondioximato ligands. The compounds crystallize in columnar stacks with different angles of inclination between the molecular planes and the stacking direction. The metal-metal distances depend strongly on the electronic and steric properties of the ligands. Upon oxidation with molecular iodine mixed valence compounds can be obtained. The stacking direction becomes perpendicular to the molecular planes, in these solids and the metal-metal distances decrease considerably. I -anions are incorporated into the lattice to form linear arrays parallel to the metal chains (1). 

We bave previously prepared and described binuclear orthometallated complexes of palladium(II) which could be used as precursors to prepare potoitially mixed-valence compounds. Hie presence of unusual Pd-N amido bonds in these complexes adds further interest based cm the reactivity shown by amides of the platinum group metals. ... 

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