Noble gases complexes

Transition Metal-Noble Gas Complexes D. C. Grills and M. W. George... 

Table 7.8 Calculated BDEs (kcal/mol) of noble gas complexes (CO)sM-Ng (Ng = Ar, Kr,Xe).a...

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

The main reasons for using an SCF as a reactant in a reaction is either to avoid using an additional solvent or to maximize the concentration of reactant. Examples include the formation of C2H4 complexes, [11,15] C-H activation, [16,17] transient formation of noble-gas complexes [18,19] and the activation of CO2 itself [3,20,21], Apart from CO2, these reactions have all been carried out on a very small scale, often without isolation of solid... 

FIGURE 15. Computed <529Si chemical shifts and geometries of Me3Si+ (26) and noble gas complexes, 28-30. Calculated NMR data at IGLO/BII//MP2(fc)/6-31G versus TMS. Reproduced by permission of Wiley-VCH from Reference 6... 

These complexes are transient species generated from photolysis of M(CO)6 in supercritical noble gas solution at room temperature. Their octahedral structure (symmetry C4V) has been characterized by IR spectroscopy and solution NMR. The noble gas atom E can be formally considered as a neutral two-electron donor ligand, and the bonding involves interactions between the p orbitals of E and orbitals on the equatorial CO groups. The stabilities of the complexes decrease in the order W > Mo Cr and Xe > Kr > Ar. Organometallic noble gas complexes... 

O. S. Jina, X. Z. Sun and M. W. George, Do early and late transition metal noble gas complexes react by different mechanisms A room temperature time-resolved infrared study of (z)5-C5H5)Rh(CO)2 (R = H or Me) in supercritical noble gas solution at room temperature. Dalton Trans., 1773-8 (2003). 

Despite the fact that a transition metal-noble gas complex has been isolated only very recently, the study of nohle gas coordination of transition metals actually has a long history. Early experiments used the technique of matrix isolation 18). Under the cryogenic conditions of frozen inert matrices, highly reactive photoproducts become sufficiently long-lived to allow their detection at leisure by conventional spectroscopic techniques such as UV/visible, IR, and EPR spectroscopy. 

Matrix isolation spectroscopy has proved an invaluable technique for the isolation and characterization of transition metal—noble gas complexes (see Table III). However, this technique has obvious limitations. Although photoproducts in low-temperature matrices can be made to react with added dopants, it is impossible to accurately predict their reactivity and mechanisms in solution at room temperature. Therefore, in the years following the original discovery of transition metal-noble gas interactions in matrices, new techniques have been used to probe these species in solution, gas phase, and supercritical fluids. 

Spectroscopic Data Used to Confirm the Formation op Transition Metal-Noble Gas Complexes in Matrix-Isolation Experiments... 

Gas-phase studies have not been restricted to the group VI hexacar-bonyls. Fu and co-workers (54) have used TRIR to study the coordina-tively unsaturated species CpMn(CO) (x = 1 and 2) generated by 266-and 355-nm laser photolysis of CpMn(CO)3 in the gas phase. In the presence of noble gas L (L = He, Ar, or Xe), they were able to measure the rate constant for reaction of the noble gas complex CpMn(CO)2L with CO. Interestingly, they foimd that only Ar significantly perturbed the rate fi om that observed in the absence of noble gas. This was thought to be because He has too high an ionization potential and Xe is too bulky to interact with the Mn center. In light of recent TRIR experiments conducted in supercritical fluid solution, the conclusion that Xe is unable to coordinate is incorrect. 

Fig. 14. Values of In (A2) ( 2 = bimolecular rate constant) for the reaction of a series of group VI noble gas complexes with CO in supercritical noble gas solution at 298 K, representing their relative stabilities. The data were taken from Ref. (64).

The Bimoleculae Rate Constants for the Reaction of a Series OF Transition Metal-Noble Gas Complexes with CO in Supercritical Noble Gas Solution at 298 K... 

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