Krypton ligand

The possibility of i) -H2 end-on coordination has also been mooted. For example, deposition of Pd atoms onto a krypton matrix doped with H2 at 12K apparently yields both Pd(t) -H2) and Pd( ) -H2) species, whereas with a Xe/H2 matrix only Pd(t) -H2) was obtained.Again, the complex [ReCl(H2)(PMePh2)4] appears to feature an asymmetrically-bonded H2 ligand which may well be ( ) -H2). ... 

The effective atomic number rule (the 18-electron rule) was described briefly in Chapter 16, but we will consider it again here because it is so useful when discussing carbonyl and olefin complexes. The composition of stable binary metal carbonyls is largely predictable by the effective atomic number (EAN) rule, or the "18-electron rule" as it is also known. Stated in the simplest terms, the EAN rule predicts that a metal in the zero or other low oxidation state will gain electrons from a sufficient number of ligands so that the metal will achieve the electron configuration of the next noble gas. For the first-row transition metals, this means the krypton configuration with a total of 36 electrons. 

A special type of TM ligands are the noble gas atoms argon, krypton, and xenon [61]. Although they are weak Lewis bases, TM complexes M(CO)sNg with M = Cr, Mo, W and Ng = Ar, Kr and Xe have been experimentally investigated in the gas phase as well as in the liquid phase and in supercritical C02 [62, 63], The M-Ng BDEs were estimated with... 

Beside XeOs and Xe04, some xenon oxyfluorides are also thermally unstable compounds. They may detonate, particularly at higher temperatures. Other derivatives of the xenon and krypton fluorides involving ligands less electronegative than fluorine should also be assmned to be of low thermodynamic stability. Many derivatives such as perchlorates and trifluoroacetates are known to be explosive. 

Within a year of the discovery of XePtFe and as a result of worldwide activity, it was clear that the chemistry of the noble gases would be limited to the heavier elements as set out in my Noranda Lecture (see Ref. S2). Because of the dangerous radioactivity associated with all of the radon isotopes, this meant that the bulk of noble-gas chemistry would be that of xenon. The chemistry of krypton appeared to be limited to KrF2 and compounds that could be derived from it. In all cases, it was clear, the range of accessible noble-gas chemistry was dictated by lower ionization potentials at the noble-gas atom, and high electronegativity and small size of the ligand atoms, as discussed in Ref. 45. 

There are two predominant challenges to direct observation of alkanes coordinated to transition metals (1) the short-lived nature of metal/alkane complexes and (2) competition for coordination of the alkane to the metal center. Because of the weak binding energy, alkane coordination is typically short-lived. Thus, fast spectroscopy techniques are required, and these techniques are often coupled with low temperatures in order to slow processes that result in alkane dissociation. In addition to the rapid dissociation of alkanes, most organic substrates will effectively compete (kinetically and thermodynamically) with alkanes for coordination to metals. Thus, the reaction medium is an important consideration since most common solvents are better ligands than alkanes, and attempts to observe alkane coordination have been commonly performed in the gas phase, in hydrocarbon matrices, or in liquid krypton or xenon. Finally, photolysis is generally required to dissociate a ligand at low temperature to create a transient coordination site for the alkane. 

Use of the teflate ligand permitted the synthesis of the first species containing a krypton-oxygen bond (Sanders, J. C. P. Schrobilgen, G. J. J. Chem. Soc., Chem. Comm. 1989, 1576-1578). The synthesis involved the interaction of KrF2 and B(OTep5)3 at low temperature (-90 to -112 C) in SO2CIF as solvent ... 

In the above discussion, no mention has been made of the effective atomic number rule proposed by Sidgwick Bailey (1934). This states that the sum of the number of metallic electrons and those donated by the carbonyls (or other ligands or bonded metals) equals the number of electrons possessed by the next inert gas, in this case the 36 electrons of krypton. This rule, which has proved to be very useful in predicting the electronic and molecular structures of a variety of organometallic compounds, indicates that there should be iron-iron bonds in Fej(CO)i2, shown by the dashed bonds in Figure 3.1, and in both the isomers of [(// -C5H5)Fe(CO)2]2- The nature of these metal-metal bonds has been the subject of extensive experimental and theoretical work. 

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