Monoolefin-iron tetracarbonyls

A series of 14 monoolefin-iron tetracarbonyl compounds has been reported by Koerner von Gustorf et al. (86). 

The ready hydrogenation and isomerization of methyl oleate and palmitoleate with Fe(CO)s confirm the results of Ogata and Misono (18) with monounsaturated aliphatic compounds. In the isomerization of monoolefins Manuel (15) suggested the occurrence of equilibria involving either 7r-olefin HFe(CO)3 and a-alkyl Fe(CO)3 complexes, or TT-olefin Fe(CO)3 and 7r-allyl HFe(CO)3 complexes. The formation of olefin-iron tetracarbonyl complexes has been reported (19). The reaction of butadiene and Fe2(CO)9 has been observed to lead to the formation of butadiene-Fe(CO)4 and butadiene-[Fe(CO)4]2 complexes in which one or both double bonds are pi-bonded to the iron (16). A mechanism involving both monoene-Fe(CO)4 (I) and allyl-HFe(CO)3 complexes (II) is postulated for the isomerization of methyl oleate (Scheme II) and for its homogeneous hydrogenation. 

If monoolefin-Fe(CO)4 complexes are involved in oleate hydrogenation, similar complexes may be invoked as intermediates in the formation of diene-Fe(CO).3 complexes from linoleate (Scheme III). Iron tetracarbonyl complexes were also suggested by Pettit and Emerson (19) in the formation of conjugated diene-Fe(CO)a from nonconjugated dienes. 

With Fe(CO)5 and Fe(CO)i2 the products isolated are, with very few exceptions, diene-iron tricarbonyls. However, using Fe2(CO)9 with dienes and monoolefins it is possible to obtain olehn-iron tetracarbonyl complexes, discussion of which will be deferred until later. 

Both conjugated and nonconjugated olefins form complexes with the transition-metal carbonyls. Despite the fact that the first known complex, Zeises salt K(PtC2H4Cl3), discovered in 1827, was that of a simple olefin, complexes of monoolefins are rather limited in number. However, nonconjugated diolefins (L) react with group-VI carbonyls to form complexes of the type LM(CO)4 an example is provided by tetracarbonyl-bicyclo-(2,2, l)hepta-2,5-diene chromium (2) (Fig. 1). In contrast, the iron carbonyls... 

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