_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Transition metal carbonyls
sequence of reactions could mimic tbose occurring at metal surfaces in tbe
Fiscber-Tropscb process (p. 393).
Sucb anionic metal carbonyls are very common. Many are clusters (p. 353).
One metbod ofpreparation is to treat tbe neutral precursor witb bydroxide ion. or
ta use a reducing agent. In eitber case one CO ligand is formally replaced by an
electron pair. Hydroxide is tbougbt first to attack tbe carbonyl group, followed by
release of carbon dioxide (as HCO; or co;-) and transfer of electrons to tbe metal:
Fe(CO)!- is tbe conjugate base oftbe carbonyl bydride H 2 Fe(C0) 4 • Tbis sbows two
neutralization steps in aqueous solution witb pK 1 = 4.0 and pK 2 = 12.7. Tbe
sodium salt Na 2 Fe(C0) 4 is a useful reagent in organic cbemistry. Single crystal X­
ray diffraction of Nale(C0) 4 • f dioxan reveals six coordinate Na + ions wbicb
interact witb tbe oxygen atoms of adjacent Fe(CO)~- units. Similar ion pairing
tbrougb Na ···O contacts also occurs in solvents sucb as tetrabydrofuran.
Otber controlled syntbeses afford Fe 2 (CO)~- from Fe 2 (C0) 9 and Fe 3 (COli; from
Fe 3 (C0) 12 :
Fe 2 (C0) 9
KOH;
Et 4 N+I-
[Et 4 Nl,[Fe 2 (CO).] (red-orange crystals)
In contrast to Co 2 (C0) 8 • Fe 2 (CO)~- bas no bridging carbonyl ligands. In
crystalline [PPNL[Fe 2 (C0) 8 l tbe ion adopts a staggered D 3 d conformation vCO
1920, 1852cm- 1 : v(Fe-Fe). 170cm- 1 (Raman). Fe 3 (COli; bas one face
bridging and one edge bridging CO group. Tbe structure is related to tbat of
Fe 4 (CO)i; by removal of tbe face bridging Fe(C0) 3 group (Fig. 5.14).
A laboratory preparation ofFe 3 (C0) 12 entails treatment ofFe(CO), witb alkali
ta give HFe(CO);. oxidation witb fresbly prepared Mn0 2 , followed by acidification.
It is likely tbat tbe reaction proceeds via HFe 1 (C0); 1 •
(c) REDUCTION TO CARBONYL ANIONS. In tbe last Section we saw tbat a
carbonylligand in a neutral metal carbonyl can often be replaced by an electron
pair by reduction. Cr(C0) 6 , bowever, can be reduced electrocbemically or by
using tbe grapbite intercalation compound C 8 K in THF to tbe dimeric anion
Cr 2 (CO)i;; tbat is, by a one electron reduction followed by coupling of tbe
resulting Cr(CO); · radicals. Furtber reduction witb sodium in liquid ammonia
affords Cr(CO)~-:
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