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I read somewhere that Fe does not react with formic acid. Because carboxylic acids are very weak. But by searching I found an article about carboxylate on wiki. It mentions about carboxylate metal salts. So, is there carboxylate of Iron? What is its chemical formula and how is it prepared?

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The formula of carboxylate ion is $\ce{RCOO-}$ where R=alkyl group or hydrogen. Iron forms two types of ion $\ce{Fe^2+}$ and $\ce{Fe^3+}$. So the general formula for iron carboxylate would be $\ce{Fe(RCOO)2}$ or $\ce{Fe(RCOO)3}$. Here I will discuss iron acetates(R=CH3-)and iron formate(R=H-).

  1. Iron(II) acetate

Iron(II) acetate is an coordination complex with formula $\ce{Fe(CH3COO)2}$. It is a white solid, although impure samples can be slightly colored. A light green tetrahydrate is also known, which is highly soluble in water.

Iron powder reacts with hot acetic acid to give the product.

$$\ce{Fe + 2CH3COOH → Fe(CH3COO)2 + H2}$$

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Actually It is not a salt. It adopts a polymeric structure with octahedral Fe(II) centers bridged by acetate ligands.

  1. Iron(III) acetate

Ferric acetate is the coordination compound more commonly known as basic iron acetate. With the formula $\ce{[Fe3O(OAc)6(H2O)3]OAc}$ where $\ce{OAc− = CH3COO−}$, it is a salt, composed of the cation $\ce{[Fe3(μ3-O)(OAc)6(H2O)3]+}$ and an acetate anion. The formation of the red-brown complex has long been used as a test for ferric ions.

Basic iron acetate is produced by treating aqueous solutions of iron(III) salts with acetate salts.Another process is solid iron is mixed with hydrogen peroxide to form iron(II/III) hydroxide, which then react with vinegar/acetic acid or acetate salts to form iron(III) acetate.

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Early work showed that it is trinuclear. The Fe centres are equivalent, each being octahedral, being bound to six oxygen ligands, including a triply bridging oxide at the center of the equilateral triangle. The compound was an early example of a molecular compound of iron that features an oxide ligand. Ignoring its 24 hydrogen centres, the cation has D3h symmetry.

  1. Iron formate

    • Iron(II) formate dihydrate($\ce{Fe(HCOO)2.2H2O}$)

Prepared by the double decomposition reaction between ferrous sulfate and sodium formate in presence of an acid.

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  • Iron(III) formate dihydrate($\ce{Fe(HCOO)3.2H2O}$)

Iron(III) formate dihydrate is obtained from freshly precipitated iron(III) hydroxide dissoluted in excess formic acid, and subsequent crystallization by evaporation in vacuum at 35 C.

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  • $\begingroup$ Excellent answer, very thorough. +1 $\endgroup$ – electronpusher Apr 30 '17 at 5:49
  • $\begingroup$ You put C-Fe bond in formate... $\endgroup$ – Mithoron Apr 30 '17 at 17:41
  • $\begingroup$ @Mithoron, I couldn't find any picture of ionic form of iron(II) formate $\endgroup$ – Nilay Ghosh May 1 '17 at 2:59
  • $\begingroup$ Then delete it or draw a new one, now it may be misleading. $\endgroup$ – Mithoron May 1 '17 at 14:31

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