Iron carbonyls, mass spectra

The mass spectrum of Fe2(CO)9, long assumed to be completely involatile, shows a parent ion, but the base peak is Fe2(CO) J, with a structure retaining the three bridging carbonyl groups [Fe(CO)3Fe]+ (70). Similar bridged ions containing iron have been postulated in the spectra of some phosphine and sulfide complexes (Section VI). 

In some polynuclear metal carbonyls of the first row (3d) transition metals the metal-metal bonds are too weak to survive complete loss of carbonyl groups. Thus in the mass spectrum of Fe3(CO)i2 stepwise loss of carbonyl groups occurs only as far as the tricarbonyltriiron ion Fe3(C0)s 2fl>. The rupture of the iron-iron bonds competes with the stepwise loss of carbonyl groups givingions such as Fe2(CO)4 and Fe(CO)4. The mass spectrum of Fe2(CO)9 exhibits the ion Fe(CO) which may represent pentacarbonyliron formed by the following pyrolysis process25) ... 

Its mass spectrum shows the molecular ion and peaks corresponding to successive loss of seven carbonyl groups plus an intense peak at m/e 112, suggesting the presence of an iron-iron bond 13). 

Recording the mass of the moleeular ion as accurately as possible can be invaluable in proving the identity of a compound. Consider a carbonyl complex of iron that gives a molecular ion at 504 Da. As iron has a mass of 56 Da, that total mass can be fitted equally well by the formulae [Fe(CO)ie] (admittedly somewhat unlikely ), [Fe2(CO)i4], [Fe3(CO)i2], [Fe4(CO)io], [Fe5(CO)g] and so on. But with high-resolution instruments it is possible to determine the masses of ions to within 0.0001 Da, and as atomic masses are not exact integers, it is usually possible to distinguish between the various options. In our iron carbonyl example, the exact masses are 503.85366,503.79876,503.74386,503.68896, and 503.63406 Da, respectively, so it is easy to decide which formula is correct. It is always a good idea to measure the mass of the parent ion in a spectrum accurately, to check that the proposed formula is correct. 

Substituted iron carbonyl complexes ineluding Fe(C0)3 As(C6H40Me-/>)3)2 and Fe(C0)4 As(C6H40Me-/>)3) were examined by electrospray mass spectrometry (ESMS). Whereas the related compound Fe(CO>3(AsPh3)2 did not give an ESMS spectrum, the methoxy-substituted compounds did. This was attributed to the presence of protonatable methoxy groups. Both the monosubstituted and disubstituted compounds were observed in a crude... 

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