As Fe(II) is a common oxidation state for Fe, many organoiron(II) compounds are known. Fe(I) compounds often feature Fe-Fe bonds, but exceptions occur, such as [Fe(anthracene)2]−. :
Ferrocene and its derivatives The rapid growth of organometallic chemistry in the 20th century can be traced to the discovery of
ferrocene, a very stable compound which foreshadowed the synthesis of many related
sandwich compounds. Ferrocene is formed by reaction of
sodium cyclopentadienide with
iron(II) chloride: :2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl Ferrocene displays diverse reactivity localized on the cyclopentadienyl ligands, including Friedel–Crafts reactions and lithation. Some electrophilic functionalization reactions, however, proceed via initial attack at the Fe center to give the bent [Cp2Fe–Z]+ species (which are formally Fe(IV)). For instance, HF:PF5 and Hg(OTFA)2, give isolable or spectroscopically observable complexes and , respectively. Ferrocene is also a structurally unusual scaffold as illustrated by the popularity of ligands such as
1,1'-bis(diphenylphosphino)ferrocene, which are useful in catalysis. Treatment of ferrocene with aluminium trichloride and benzene gives the cation [CpFe(C6H6)]+. Further
iron arene complexes are also possible. Oxidation of ferrocene gives the blue 17e species
ferrocenium. Derivatives of
fullerene can also act as a highly substituted cyclopentadienyl ligand.
Fp2, Fp−, and Fp+ and derivatives Fe(CO)5 reacts with
cyclopentadiene to give the dinuclear Fe(I) species
cyclopentadienyliron dicarbonyl dimer ([FeCp(CO)2]2), often abbreviated as Fp2. Pyrolysis of Fp2 gives the cuboidal cluster [FeCp(CO)]4. Very hindered substituted cyclopentadienyl ligands can give isolable monomeric Fe(I) species. For example, Cpi-Pr5Fe(CO)2 (Cpi-Pr5 = i-Pr5C5) has been characterized crystallographically. Reduction of Fp2 with sodium gives "NaFp", containing a potent
nucleophile and precursor to many derivatives of the type CpFe(CO)2R. The derivative [FpCH2S(CH3)2]+ has been used in
cyclopropanations. The Fp+ fragment is Lewis acidic and readily forms complexes with ethers, amines, pyridine, etc., as well as alkenes and alkynes in the η2 coordination mode. The complex Fp+(η2-
vinyl ether)]+ is a masked
vinyl cation. Recently, a
methane complex, [Fp(CH4)]+[Al(OC(CF3)3)4]–, was prepared and characterized spectroscopically, using a perfluoroalkoxyaluminate as a non-coordinating counterion and 1,1,1,3,3,3-hexafluoropropane as a non-coordinating solvent. Fp-R compounds are
prochiral, and studies have exploited the chiral derivatives CpFe(PPh3)(CO)acyl.
Alkyl, allyl, and aryl compounds is a rare example of a neutral per-organo complex of iron The simple peralkyl and peraryl complexes of iron are less numerous than are the Cp and CO derivatives. One example is
tetramesityldiiron. Compounds of the type [(η3-allyl)Fe(CO)4]+X− are
allyl cation synthons in
allylic substitution. Similarly, allenyl(cyclopentadienyliron) dicarbonyl complexes exhibit reactivity analogous to main group allenylmetal species and serve as nucleophilic propargyl synthons.
Sulfur and phosphorus derivatives Complexes of the type
Fe2(SR)2(CO)6 and Fe2(PR2)2(CO)6 form, usually by the reaction of thiols and secondary phosphines with iron carbonyls. The thiolates can also be obtained from the tetrahedrane
Fe2S2(CO)6. ==Iron(III)==