Coordination compounds

Key points: Argon, xenon, and krypton form coordination compounds that are usually studied by matrix isolation; the stability of the complex es decreases in the order Xe Kr Ar. Coordination compounds of noble gases have been known since the mid-1970s. The first stable noble-gas coordination com pound to be synthesized was [AuXe₄]⁺² [SbF11]^-2, which contains a square-planar [AuXe₄]⁺² cation (21). The compound is made by reduction of AuF₃ by HF/SbF₅ in elemental xenon to yield dark red crystals that are stable up to 78°C. Alternatively, addition of Xe to a solution of Au2 in HF/SbF₅ gives a dark red solution that is stable up to 40°C. This solution is stable at room temperature under a xenon pressure of 1 MPa (about 10 atm). During the reduction of Au3 to Au2 the extreme Brønsted acidity of HF/SbF5 (Section 15.11) is essential and the overall reaction indicates the role of protons:

Green crystals of [Xe₂]⁺ [Sb₄F₂₁]^- are also produced at 60°C. The Xe-Xe bond length is 309 pm and is the longest homo nuclear bond known for a main-group element.

Many complexes of the noble gases are transient species that have been characterized by matrix isolation. The complex Fe (CO)₄ Xe is formed when Fe(CO)₅ is photolysed in solid xenon at 12 K. Similarly, M(CO)5 E is formed when M(CO)₆ (M Cr, Mo, or W) is photolysed in solid argon, krypton, or xenon at 20 K, where E=Ar, Kr, or Xe, respectively. An alternative method of synthesizing these complexes is to generate them in an argon, krypton, or xenon atmosphere. The Xe complex has also been isolated in liquid xenon. The stabilities of the complexes decrease in the order W>Mo ≈ Cr and Xe>Kr>Ar. The complexes are octahedral (22) and the bonding is thought to involve inter actions between the p orbitals on the noble gas and orbitals on the equatorial CO groups. Thus, noble gases can be thought of as potential ligands, and indeed have been fully characterized as such (including by NMR; Section 22.16).

When [Rh(n⁵-Cp) (CO)₂] or [Rh(n⁵-Cp*) (CO)₂] is photolysed in supercritical xenon or krypton at room temperature, the complexes [Rh(n⁵-Cp) (CO)E] and [Rh(n⁵-Cp*) (CO)E] are formed, where E Xe or Kr. The n⁵-Cp* complexes are less stable than the n⁵-Cp analogues and the Kr complexes are less stable than those of Xe.

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مواضيع ذات صلة

Other compounds of noble gases

Organoxenon compounds

Xenon oxygen compounds

Reactions of xenon fluorides

Synthesis and structure of xenon fluorides

Occurrence and recovery

Hydrogen peroxide

Water

Selenium, tellurium, and polonium

Sulfur

Reactivity of oxygen

Oxygen