In chemistry, compounds of palladium(III) feature the noble metal

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself nam ...

in the unusual +3

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...

(in most of its compounds, palladium has the oxidation state II). Compounds of Pd(III) occur in mononuclear and dinuclear forms. Palladium(III) is most often invoked, not observed in mechanistic organometallic chemistry.

Mononuclear compounds

Pd(III) has a d⁷

electronic configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom ...

, which leads to a Jahn-Teller distorted

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

geometry. The geometry could also be viewed as being intermediate between square-planar and octahedral. These complexes are low-spin and

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...

. The first Pd(III) complex characterized by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angle ...

was reported in 1987. It was obtained by oxidation of the

1,4,7-trithiacyclononane 1,4,7-Trithiacyclononane, also called 9-ane-S3, is the thia-crown ether with the formula (CH2CH2S)3. This cyclic thioether is most often encountered as a tridentate ligand in coordination chemistry, where it forms transition metal thioether compl ...

(ttcn) complex d(ttcn)₂sup>3+. X-Ray crystallography revealed the expected Jahn-Teller distorted octahedral geometry, in spite of the highly symmetric structure of the

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

. The first

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and ...

Pd(III) complex characterized by X-Ray crystallography was reported in 2010. Organopalladium complexes supported with a macrocyclic tetradentate ligand undergo single-electron oxidation to give Pd(III) species that is stabilized by the axially-positioned amine. The authors propose that while the axial nitrogen stabilize a distorted octahedral geometry, the t-Bu group and the rigidity of the macrocyclic structure inhibits the oxidation to a more conventional octahedral Pd(IV).

Dinuclear compounds

Structure

Pairs of Pd(III) centers can couple, giving rise to a Pd–Pd

bond order In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs ( covalent bonds) between two atoms. For example, in diat ...

of 1. A two-electron oxidation of two Pd(II) species can generate a diamagnetic, Pd(III)-Pd(III) dimer with a bond order of 1. Palladium(III)6

The first example of a dipalladium(III) complex was obtained by oxidation of dinuclear Pd(II) complex of triazabicyclodecene. The first organometallic dinuclear Pd(III) complexes were reported in 2006 by Cotton and coworkers as well. These complexes catalyze the diborylation of terminal olefins. Due to the facile reduction of these complexes to Pd(II) species by diborane, the authors proposed that the dinuclear Pd(III) complexes serve as precatalysts for active Pd(II) catalysts.

Reactivity

The reactivity of dinuclear Pd(III) species as active catalytic intermediate is mostly discussed in the context of C-H activation. While it was proposed that Pd-catalyzed oxidative C-H functionalization reactions involve a Pd(IV) intermediate, Ritter and coworkers first postulated that these oxidative reactions could involve a dinuclear Pd(III) intermediate instead of Pd(IV). Dinuclear Pd species are involved in Pd-catalyzed C-H chlorination. Through X-ray crystallography, Ritter unambiguously showed that dinuclear Pd(III) complex is formed when the palladacycle is treated with two-electron oxidant, and such dinuclear complex undergoes C-Cl reductive elimination under ambient temperature. Both experimental and computational data was consistent with a concerted 1,1-reductive elimination mechanism for the C-Cl forming step. The authors show that such bimetallic participation of redox event lowers the activation barrier for reductive elimination step by ~30 kcal/mol compared to a monometallic pathway. Acetoxylation of 2-phenylpyridine was also demonstrated to involve a dinuclear Pd(III) intermediate.

References

{{Chemical compounds by element Palladium compounds P