Xenon complexes

The compounds (NO)2[PdF6]319 and [XeFs PdFj320 have been reported. A structural determination on the latter compound showed bond lengths of 1.89 A (Pd—F).320 It has been suggested that the xenon complex XePd2Fi0 is in fact an XeF+ salt of [Pd ] -.307... 

Before leaving the NgMX complexes and the compound HArF introduced in Section 5.8.1, it is noted that a fuller discussion on the chemistry of noble gases may be found in Section 17.5, including a subsection on gold-xenon complexes in Section 17.5.6. 

Xenon can act as a complex ligand to form M-Xe bonds, especially with gold, which exhibits significant relativistic effects in view of its electronic structure, as discussed in Section 2.4.3. Some gold-xenon complexes have been prepared and characterized, and their structures are shown in Fig. 17.5.7. 

The remarkable xenon complex Cr(CO)5(Xe) was also obtained by UV photolysis of Cr(CO)6 in liquefied xenon and found to have a lifetime of ca. 2 s at -98°C [117]. Later, the corresponding molybdenum and tungsten complexes Mo(CO)5(Xe) and W(CO)5(Xe) were generated in a similar way and characterized by time-resolved IR spectroscopy [118]. The bond energies Cr-Xe, Mo-Xe, and W-Xe were not as low as previously anticipated and determined to be 8-9 kcal/mol, independent of the group VI metal. 

Similar TRIR experiments were performed with Tp Rh(CO)2 and Bp Rh(CO)2 [Tp = hydridotris(3,5-dimethylpjn azolyl)borate Bp = dihydridobis(3,5-dimethyIpyrazolyl)borate] in liquid Xe solution at 223 K 48). From the positions of the observed transient infrared bands, it was found that the Tp species forms two xenon complexes upon photolysis, (7j -Tp )Rh(CO)Xe and (>j -Tp )Rh(CO)Xe, and that Bp Rh(CO)Xe is produced from Bp Rh(CO)2. 

A slight deficiency of CsF mixed with [Xe,F,i]+ -AuF,]-was heated under dry N, to 110 °C, at which temperature XeF, evolved. The XeF, displacement CsF -)- [Xe,F ]- [AuF,] ->- 2XeF, + CsAuF, was completed under vacuum, (the slight excess of xenon complex sublimed out of the reactor) and a pale yellow solid remained. X-Ray powder photographs established that CsAuF, is isomorphous with the other CsMF, noble metal salts. Crystal data CsAuF,i M = 443-9 rhombohedral, a — 5-24 i 0-01 A a = 96-5 0-3°, V = 141 A, 2=1, Z c = 5-22 g cm-. Space... 

Key properties of noble gases are summarized in Table 1 other properties are in refs. 6 and 11. The formation and detection of M-Ng bonds provide many of the same challenges as for transition metal-alkane bonds. Comparison of spectra and energetics of species such as Cr(CO)5Xe and Cr(CO)s(alkane) points to further similarities in bond enthalpies. Recent reviews tackle complexes of alkanes in detail . Since investigations of alkane complexes are more frequent than those of xenon complexes, we may use these results to anticipate the formation of further M-Ng bonds [e.g., to HMn(CO)4]. 

One other system investigated in detail by TRIR is the photolysis of Rh(> -C5Mes)(CO)2 in Kr(l) and Xe(l). Both the TRIR technique with an IR laser and the method with the FTIR spectrometer synchronized to the pump laser pulse have been reported. The photoproducts are assigned as Rh(> -C5Me5)(CO)Ng on the basis of their IR spectra in the carbonyl-stretching [v(CO)] region (a single band at ca. 1945 cm ) and their reactivity toward CO and alkanes. The krypton complex proves to be far more reactive than the xenon complex even at lower temperature. Toward CO, there is a difference in reactivity of a factor of ca. 200 at comparable temperatures ... 

The methodology for studying M-Ng complexes in the gas phase is essentially the same as the TRIR method for liquified noble gases a pump pulse photolyzes a metal carbonyl ion and the fragment is detected with the aid of a continuous IR laser. In these experiments helium is utilized as the standard buffer gas. A xenon complex may be detected by alteration in the spectrum and kinetics on addition of xenon. Since the spectra are free of solvent effects, the effect of coordination should be more easily discerned than in the liquid phase. This method has been used to study M(CO)sXe (M = Cr, Mo, W) and W(CO)sKr. Metal-xenon bond energies of ca. 35 kJmol are deduced from the kinetics of reaction with CO. The variation between metals in comparable to the error bars, about 4 kJ mol . The W-Kr bond energy is estimated to be less than 25 kJmol . ... 

Alternatively, an t/ -interaction is possible, but later studies of a pentane complex show that, as supported calculation-ally, binding is primarily through one C-H bond at any one instant. This bond rapidly exchanges with other bonds in the CH3 or CH2 unit that is attached to Re, with a slight preference for methylene C-H. 7ch fur the bound C-H is 85 Hz versus 132 Hz in the unbound C-H. A related xenon complex was observed in liquid xenon. 

K. Seppelt (2003) Zeitschrift fur anorganische und allgemeine Chemie, vol. 629, p. 2427 - Metal-xenon complexes . 

---