Palladium compounds, electronic structure transition state

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). 

Palladium(O) compounds have a configuration and unlike most transition metals this oxidation state is dominated by phosphine complexes rather than carbonyls. In fact binary carbonyl complexes with palladium are unstable at room temperature. The highest coordination number known for Pd is four and [PdLj complexes adopt a square planar structure. Dissociation of ligands from [PdLJ occurs readily to generate the 16- and 14-electron species [PdLs] and [Pdl ] these are trigonal planar and linear respectively. Another notable feature of Pd° is that facile oxidation to cr Pd occurs. 

---