Trans-spanning ligands are bidentate

ligands In coordination chemistry, a ligand is an ion or molecule with a 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 ...

that can span opposite sites of a complex with square-planar geometry. A wide variety of ligands that chelate in the cis fashion already exist, but very few can link opposite vertices on a coordination polyhedron. Early attempts to generate trans-spanning bidentate ligands relied on long hydrocarbon chains to link the donor functionalities, but such ligands often lead to

coordination polymer Coordination may refer to: * Coordination (linguistics), a compound grammatical construction * Coordination complex, consisting of a central atom or ion and a surrounding array of bound molecules or ions ** A chemical reaction to form a coordinat ...

History

A diphosphane linked with

pentamethylene Cyclopentane (also called C pentane) is a highly flammable alicyclic hydrocarbon with chemical formula C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It is a ...

was claimed to span across a square planar complex. This early attempt was followed by ligands with more rigid backbones. "TRANSPHOS" was the first trans-spanning diphosphane ligand that usually coordinates to palladium(II) and platinum(II) in a trans manner. TRANSPHOS features benzo henanthrene substituted by diphenylphosphinomethyl (Ph₂PCH₂) groups at the 1 and 11 positions. The polycyclic framework suffers sterically clashing hydrogen centers.

SPANphos, TRANSDIP, and related ligands

TRANSDIP, based on a α-

cyclodextrin Cyclodextrins are a family of cyclic oligosaccharides, consisting of a macrocycle, macrocyclic ring of glucose subunits joined by α-1,4 glycosidic bonds. Cyclodextrins are produced from starch by enzyme, enzymatic conversion. They are used in ...

, is the first ligand to give exclusively trans-spanned complexes, even with d⁸ metal ion halides.

Xantphos Xantphos is an organophosphorus compound derived from the heterocycle xanthene. It is used as a bidentate diphosphine ligand and is noteworthy for having a particularly wide bite angle (108°). Such ligands are useful in the hydroformylation of ...

is sometimes classified as a trans-spanning ligand, with less steric bulk compared to TRANSPHOS.

SPANphos SPANphos is an organophosphorus compound used as a ligand in organometallic chemistry, organometallic and coordination chemistry. The compound is a rare example of a trans-spanning ligand and rigidly links mutually trans coordination sites. By vir ...

is comparable to XANTPHOS but more reliably trans-spanning.

References

{{reflist Coordination chemistry