Palladium compounds, electronic structure

PdF2 is that rare substance, a paramagnetic palladium compound, explicable in terms of (distorted) octahedral coordination of palladium with octahedra sharing corners [15], It exists in two forms, both having /zeff 2.0 /xB, rather below the spin only value for two unpaired electrons. Bond lengths are Pd-F 2.172 A (two) and 2.143 A (four) in the tetragonal form (rutile structure). 

In a sense the tr-allyl compounds of the transition metals can be regarded as the simplest of the sandwich molecules. Bis(jr-allyl)nickel, the best known of such complexes, has been shown by x-ray crystallography (104,105) to have a staggered arrangement of tr-allyl moieties and hence a C2h molecular conformation. The electronic structure of the ground state of bis(jr-allyl)nickel has been investigated by both semiempirical (47) and ab initio (274,275) methods, and a semiempirical computation has been performed on bis(7r-allyl)palladium (47). 

Tungsten carbide — WC, belongs to a class of Group IV B-VIB transition metal carbides and nitrides, often referred to as interstitial alloys, in which the carbon and nitrogen atoms occupy the interstitial lattice positions of the metal [i]. These compounds possess properties known from group VIII B precious metals like platinum and palladium [ii]. Thus, they show remarkable catalytic activities, attributed to a distinct electronic structure induced by the presence of carbon or nitrogen in the metal lattice. Tungsten carbide resembles platinum in its electrocatalytic oxidation activity (- electrocatalysis) and is therefore often considered as an inexpensive anode electrocatalyst for fuel cell [iii] and -> biofuel cell [iv] application. 

The coordinative unsaturation of the palladium compound follows directly from its structure. The compound (7r-C5H5)TiCl3 (coordination number 6) has a so-called piano stool structure and reactivity towards an electron donor molecule is probably due to the vacant nonbonding titanium orbitals 215, 216). 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Palladium chemistry involving heterocycles has its unique characteristics stemming from the heterocycles inherently different structural and electronic properties in comparison to the corresponding carbocyclic aryl compounds. One example illustrating the striking difference in reactivity between a heteroarene and a carbocyclic arene is the heteroaryl Heck reaction (vide infra, see Section 1.4). We define a heteroaryl Heck reaction as an intermolecular or an intramolecular Heck reaction occurring onto a heteroaryl recipient. Intermolecular Heck reactions of carbocyclic arenes as the recipients are rare [12a-d], whereas heterocycles including thiophenes, furans, thiazoles, oxazoles, imidazoles, pyrroles and indoles, etc. are excellent substrates. For instance, the heteroaryl Heck reaction of 2-chloro-3,6-diethylpyrazine (1) and benzoxazole occurred at the C(2) position of benzoxazole to elaborate pyrazinylbenzoxazole 2 [12e]. 

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