Osmium compounds spectra

This is considerably different from the recombination reaction with, for example, typical ruthenium dyes. This slow re-reduction of the dyad is explained by the low redox potential of the osmium center, the value of 0.66 V (vs. SCE) observed, points to a small driving force for the redox process. This observation is important for the design of dyes for solar cell applications. Osmium compounds have very attractive absorption features, which cover a large part of the solar spectrum. However, their much less positive metal-based oxidation potentials will result in a less effective re-reduction of the dyes based on that metal and this will seriously affect the efficiency of solar cells. In addition, for many ruthenium-based dyes, the presence of low energy absorptions, desirable for spectral coverage, is often connected with low metal-based redox potentials. This intrinsically hinders the search for dyes which have a more complete coverage of the solar spectrum. Since electronic and electrochemical properties are very much related, a lowering of the LUMO-HOMO distance also leads to a less positive oxidation potential. 

Osmium compounds appear in various guises in this report. At the high oxidation state end of the spectrum, the Os(IV) complex OsH3(SiMe3)(CO)(PPh3)2 has been synthesised by Mohlen and co-workers. Lower down the oxidation ladder, the tetraosmium complex [Os2(CO)5(thd)2]2 has been synthesised (and an EHMO study included). 

F.13 Osmium forms a number of molecular compounds with carbon monoxide. One light-vellow compound was analyzed to give the following elemental composition 15.89% C, 21.18% O, and 62.93% Os. (a) What is the empirical formula of this compound (b) From the mass spectrum of the compound, the molecule was determined to have a molar mass of 907 g-mol 1. What is its molecular formula ... 

The pentanuclear carbido species Ms(CO)lsC (M = Fe, Ru, Os) have been prepared. The iron compound has been known for some considerable time (209), but the ruthenium and osmium complexes were prepared recently by pyrolysis reactions (210). The ruthenium adduct was only isolated in low yield (—1%), while the osmium complex was obtained in higher yield (—40%). The infrared spectrum and mass spectral breakdown pattern indicate a common structure to these compounds. The molecular structure of the iron complex is shown in Fig. 46. 

This compound is the only one known for these elements in this oxidation state it is obtained as a highly-colored solid by the high-temperature reaction between fluorine and the metal. The solid is isostructural with the hexafluorides of osmium and iridium, and the Pt-F distance has been estimated at 1 83 A by extrapolation along the series W-Os-Ir 255). The infrared spectrum has been assigned in Oh symmetry 256) there are no signs of any distortion, as found for example in osmium hexafluoride. 

In their original paper (2) on the structure of Fe5C(CO)l5, Dahl and co-workers assigned two bands in the infrared spectrum of hydrocarbon solutions of the cluster, at 790 and 770 cm-1, to vFeC modes. This assignment has been confirmed by a recent study of the infrared spectra of the series M5C(CO)15, (M = Fe, Ru, Os) (78). The room temperature spectra of the compounds (Table II) in the solid state are quite similar to each other, comprising three bands assigned as the a, and e modes (split in the solid state) expected for the C4 symmetry of the isostructural clusters. At low temperature the ruthenium and osmium clusters exhibit five absorptions associated with M-C stretches, whereas the iron cluster retains its room temperature spectrum. This is ascribed to the presence of two types of cluster molecule in the crystal lattices of the ruthenium and osmium clusters which are isostructural with, but not isomorphous with, the iron analog in which all the molecules are identical. 

The reaction of osmium atoms with benzene in the vapour phase produces the yellow compound [OsC H J. The signals appearing in its 1H spectrum are listed below, and the lH 1H exchange spectrum of the compound is shown on the right. What exchange process is occurring ... 

Some analogous rathenium- and osmium-bismuth clusters have been found. Examples include Bi2M3(CO)9 and H3BiM3(CO)9 (M = Ru, Os). The stmctures of the hydride compounds have both been determined and they are isostractural with the iron complexes as is Bi2Ru3(CO)9 withBi2Fe3(CO)9. The structure 0fBi2Os3(CO)9, on the other hand, has not been determined and its IR spectrum indicates that it probably has a different structure. A spirocyclic cluster [Ru2(CO)8(/X4-Bi)Ru3(CO)io(/x-Ft)] (39) has been reported. 

Figure 9. Schematic energy level diagram(left) for Ru230s compounds. Evidence for the photoinduced energy transfer process can be obtained from a) the emission spectrum (quenching of the ruthenium emission and sensitization of the osmium) and b) the rise time.

This compound presents some challenges to structural chemistry. The NMR spectrum shows a single line, so all of the carbon atoms are equivalent. An ordinary X-ray structure for the molecule cannot be obtained because the molecule is rotating and hence disordered in the solid phase. Since the molecule is essentially a sphere, this is not surprising. An X-ray structure was obtained for an osmium derivative of the fullerene, however. Even more useful, a radM distribution function similar to what is normally obtained from electron diffraction experiments (Chapter 2) was obtained... 

As mentioned earlier, molecules containing chlorine or bromine produce easily characterized spectra. This observation is one of the most useful aspects of mass spectrometry for organic chemists. Table 15.1 lists the natural isotopic ratio for Cl to Cl (75.77 24.23), which is essentially a 3 1 ratio. The ratio for Br to Br (50.69 49.31) is close to a 1 1 ratio. Chlorine and bromine are the only elements frequently found in organic molecules that have significant M + 2 isotopes. There are other elements (osmium and mercury, for example) that have M -b 2 isotopes, but we are less likely to deal with mass spectra of such compounds. Therefore, if a mass spectrum shows an M -b 2 peak that is one-third the intensity of the molecular ion (M ) as shown in Figure I5.14a, then we know there is a chlorine in the molecule. If a spectrum has an M -b 2 peak that is the same intensity as the molecular ion, we can conclude that the molecule has a bromine (see Rg. 15.14b). 

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