Iron complexes, with fumaric acid

Several aUcene complexes of iron(O) have been reported, of general formula Fe(CO)4(alkene), where the aUcene can be ethylene, acrylonitrile, maleic anhydride, or methyl methacrylate. In these complexes, the original trigonal bipyramidal structure of the pentacarbonyl is retained with one of the equatorial positions now being occupied by the alkene ligand. The tetracarbonyliron(O) complex of fumaric acid has idealized C2 symmetry and has been resolved into its two enantiomers. ... 

An X-ray analysis (138) of racemic (fumaric acid)Fe(CO)4 shows a trigonal bipyramidal arrangement of the five ligands about the iron atom with the double bond at an equatorial position (63). The complex molecules are hydrogen-bonded through the fumaric acid ligands. Unlike... 

Absorption of iron by the individual varies with age, iron status, the amount and chemical form of the iron ingested, and with conditions in the gastrointestinal tract, only about 5—15% of iron in the diet being normally absorbed. Ferrous iron, as the sulphate, gluconate, fumarate or lactate or as ferrous ammonium sulphate, is appreciably taken up into the bloodstream from the duodenum, especially in the presence of ascorbic acid, a reducing agent. Little difference was found in the extent of their absorption between ferrous sulphate and the various chelates, but ferric ammonium citrate or polysaccharide complexes were only very poorly absorbed22)... 

---