Osmium compounds mass spectrum

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 ... 

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). 

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. 

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