A-frame complexes are coordination compounds that contain two bridging

Bridging ligand

A bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are...

 bidentate ligands and a single atom bridge. They have the formula M₂(μ-X)(bd)₂L₂, where bd is a bidental ligand like dppm

1,1-Bis(diphenylphosphino)methane

1,1-Bismethane , is an organophosphorus compound with the formula CH22. Dppm, a white, crystalline powder, is used in inorganic and organometallic chemistry as a ligand...

, and X and L are a wide variety of ligands. The term was coined to describe products arising from the oxidative addition to Rh(I)Rh(I) complexes.

Scope of compounds

A-frame complexes typically consist of a pair of square-planar metal centres. Consequently, this family of complexes is found for those metals that tend to adopt that geometry, Rh, Ir, Ni, Pd, Pt, and Au. In addition to dppm, the analogous tetramethyldiphosphine (dmpm) also forms such complexes as do some related ligands, such as 2-diphenylphosphinopyridine. The bridging ligand can be a variety of ligands, including CO, SO

Sulfur monoxide

Sulfur monoxide is an inorganic compound with formula . It is only found as a dilute gas phase. When concentrated or condensed, it converts to S2O2 . It has been detected in space but is rarely encountered intact otherwise.-Structure and bonding:The SO molecule has a triplet ground state similar...

, NO, CH₂, and a variety of anions such as hydride

Hydride

In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...

 and chloride.

Preparation

A frame complexes are often produced by the addition of reagents of the type AX₂ to low valent complexes of dppm:

2 M(0) + AX₂ + 2 dppm → M₂(μ-A)(dppm)₂X₂

Alternatively the group "A" can be added across a preformed M-M bond, as indicated by the oxidative addition

Oxidative addition

Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre...

of elemental sulfur:

Pd₂(dppm)₂Cl₂ + S → Pd₂(μ-S)(dppm)₂Cl₂