Transition metal arsine complexes

Transition metal arsine complexes are coordination complexes containing one or more arsine ligands. Almost always, the arsine is an organoarsenic compound of the type R₃As (R = alkyl, aryl).^[1]

Ligand properties

Structure of trans-PdCl₂(AsPh₃)₂.^[2] (Ph = C₆H₅)

Arsines are L ligands according to the Covalent bond classification method. In the usual electron counting method, they are two-electron ligands. With respect to HSAB theory, the ligand is soft.

Comparison of arsine and phosphine ligands

An obvious difference between arsine and phosphine ligands is the larger size of arsenic atoms. Consequently, M-As bonds tend to be about 10 picometers longer than M-P bonds.^[3] Furthermore, the ligand cone angle is somewhat smaller for arsines vs phosphines: 141° for AsPh₃ vs 145° for PPh₃.^[4]

The inversion barriers for tertiary arsines are estimated to be near 40 kcal/mol. By contrast, the inversion of a tertiary phosphine occurs with barriers near 30 kcal/mol. Thus, chiral arsines are more optically stable than chiral phosphines.^[5]

Arsines are more reluctant to oxidize to As(V) derivatives. For this reason, alkylarsines are more air-stable than the corresponding alkylphosphines.

Representative ligands

Symmetrical triorganoarsines are well-established, including triphenylarsine and trimethylarsine. A well studied chelating ditertiary arsine is diars, C₆H₄(As(CH₃)₂)₂. Some of these ligands were developed in the course of study on chemical warfare agents.^[6]

Synthesis and reactions

Metal arsine complexes are heavy analogues of the more widely studied metal-phosphine complexes. Arsine-halide complexes are prepared by reactions of metal halides with preformed arsines. For example, treatment of a suspension of platinum(II) chloride in ethanol with triethylarsine yields monomeric bis(triethylarsine)platinum(II) chloride.^[3] Arsine carbonyl complexes are often prepared, again like the phosphine analogues, by thermal or UV-induced displacement of CO ligands.

The reactions of metal arsine and phosphine complexes are similar. Arsines are weaker Lewis bases, so arsine ligand are more readily displaced.

As catalysts for cross-coupling reactions, palladium-arsines are sometimes superior to their phosphine analogues.^[7]