Square planar palladium

The important oxides are black PdO and brown Pt02. The former can be made by heating palladium in oxygen other methods include heating PdCl2 in an NaN03 melt at 520°C. A hydrated form precipitates from aqueous solution, e.g. when Pd(N03)2 solution is boiled. It has 4-coordinate square planar palladium (Figure 3.8). 

Reactions of RSH with MCI4- in aqueous solution lead to precipitates of the neutral thiolates M(SR)2 with small alkyl and aryl substituents, the products are oligomeric Pd(SPr )2 is hexameric with square planar palladium (Figure 3.67) [115],... 

Figure 86 Compound containing a full M2L4 helical cage that has each square-planar palladium atom coordinated to the four bridging ligands and within which resides a well-ordered PF6 ion.

In addition to the presence of these elements in ores, they are also available from recycled feeds, such as catalyst wastes, and as an intermediate bulk palladium platinum product from some refineries. The processes that have been devised to separate these elements rely on two general routes selective extraction with different reagents or coextraction of the elements followed by selective stripping. To understand these alternatives, it is necessary to consider the basic solution chemistry of these elements. The two common oxidation states and stereochemistries are square planar palladium(II) and octahedral platinum(IV). Of these, palladium(II) has the faster substitution kinetics, with platinum(IV) virtually inert. However even for palladium, substitution is much slower than for the base metals so long as contact times are required to achieve extraction equilibrium. 

Detailed investigations on the kinetics and mechanisms of reactions of square planar palladium (II) complexes are largely lacking. However, enough data exist to show that the reactions of palladium (II) complexes are much faster than those of platinum (II), and that the two systems react by the same type of mechanism. Some of the data available are given in Table VIII along with the same information on platinum (II) and nickel (II) for comparison (3). The results show an approximate relative order of reactivity for analogous complexes of the triad as follows ... 

One or more factors may be operating simultaneously to provide a delicate balance of counterpoising effects. An interesting series of compounds illustrates the competing effects in linkage isomers of square planar palladium(II) complexes (Fig. 12.46a-d).87 The six-membered chelate ring in Figure 12.46c allows an essen-... 

The square planar palladium complexes which give values of m of ca. 0.4 (Table 1) are known to react via a mainly associative mechanism so that the values of m are taken to indicate that Pd—Cl bond cleavage and leaving Cl solvation were both important in determining the reactivity trend for these complexes, i.e. there is a greater degree of M—Cl bond breaking in the transition state of palladium compared with cobalt. 

Because substitution chemistry at square-planar palladium is dominated by associative processes [48], coordination of the alkene in 22.2 would undoubtedly initially generate penta-coordinate intermediate 22.6. Complex 22.6 could then either evolve to square-planar complex 22.5 by a series of pseudorotations and eventual expulsion of the halide ligand or undergo... 

These new complexes and polymers are related to the square-planar palladium and platinum polyynes which have recently been shown (25-28) to exhibit interesting X ) behavior. We have not yet measured the %(3) properties of our rhodium complexes, or the molecular weights of the polymers. Current synthetic work is directed towards understanding the influence of the linker groups on electronic communication between the metal centers, and on designing new linkers with low-lying 7i levels to improve conjugation. 

The five-membered heterocycle 18 is nonplanar and combines square planar palladium and tetrahedral germanium centers in the same cycle <20040M2370>. 

Aside from full catalytic cycles, or reaction steps within them, as discussed above, structural issues about organometallic complexes with catalytic properties have also been analyzed with QM/MM methods. For instance, Helmchen and coworkers carried out a conformational analysis of two (w-l,3-dimethylallyl)(phosphinooxazoline)Pd complexes [128], and Magistrato et al investigated the role of n-n stacking interactions in square planar palladium complexes [129]. 

The application of this approach to phosphorus dendrimers is readily applied11111 to the creation of a tetrahedral series via the use of a four-directional silane core (Scheme 4.26). The treatment of tetravinylsilane (78) with phosphine 98 quantitatively gave the desired small dendrimer 99, which can be transformed to the tetrakis(square planar palladium) complex see Chapter 8. 

Lopez et al. [27] prepared Pd/SiC>2 catalysts under both acidic (pH = 3) and basic (pH = 9) conditions in the sol-gel step and reported that an acid medium promotes the formation of small metal crystallites. This finding is consistent with the formation of a micro-porous silica gel network at a low pH. By comparing samples prepared by the sol-gel method and impregnation, these authors found in the former a stronger metal-support interaction which they ascribed to the square planar palladium complex used as a precursor. Finally, their results showed that the method of preparation as well as the conditions used in each method impact on how these catalysts deactivate in the hydrogenation of phenylacetylene. 

Pd(ArNNNAr)2(ArNNNHAr) were claimed as the first examples of octahedral palladium(II) (77). However, subsequent attempts to prepare these compounds invariably led to bis-adducts which were formulated as square-planar palladium(II) species (9S). 

The palladium(ll) complexes were used in the Heck reaction between aryl halides and styrene using either water or DMF as the solvent. Whenever comparison is possible, the yields for the aryl bromides were significantly better than for the analogous chlorides. It may be interesting to note that the dimeric ft-chloride bridged palladium complex is not coplanar, but displays a butterfiy structure with respect to its two square planar palladium environments. 

A homoleptic square-planar palladium(II) carbonyl complex [Pd(CO)4](Sb2Fii)2 that displays v(CO) at 2259 cm (the value for free CO is 2143 cm ) has been recently prepared. This is an exceptional complex, dubbed by the authors as superelectrophilic metal carbonyl . The very high value of v(CO) shows that the carbonyl hgands behave as a-donors (see Carbonyl Complexes of the Transition Metals). Additional data (e g. v(M )) and stability data support that this ligand is very weakly coordinated as only a a-donor. ... 

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