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

Transition metal chemistry
Carbonyl ligands in nitrosyl carbonyls Mn(C0) 4 (NO), Fe(C0) 2 (N0) 2 and
Co(C0) 3 (NO) are substituted by an SN2 (A or Ia) mechanism (Fig. 5.12(d)). This
occurs because a lone pair can be localized on nitrogen in the intermediate (or
transition state). The nitrosyl group can act either as a formal three-electron
- + yO
(linear M=N=O) or as a one-electron donor (bent M-~ ). (see p. 294
for substitution of cyclopentadienyl metal carbonyls).
(b) REACTION OF HARD BASES. Pentacarbonyliron reacts with amines such as
pyridine, not by substitution, but by disproportionation. A cluster anion is
produced:
SFe(CO), + 6py
reflux
(PPN + = Ph 2 P=N=PPh 2 )
+
[Fe(py) 6 ][Fe.(C0) 13 ] + 12CO
X-ray diffraction of the PPN salt reveals a tetrahedral cluster of iron atoms
(Fig. 5 .14( c) ). One triangular face is capped by a triply bridging CO group; around
each of the sides of this same face there is an edge-bridging carbonyl. On protonation
the tetrahedral cluster opens out to a butterfly framework (cf. p. 350);
the protonated CO ligand is now a-bonded to the fourth iron atom. Further
reduction under acid conditions affords methane; it has been suggested that this
co co co
(o) oc~co
co co co
(b) oc
co
co
(c)
oc
oc
Fig. 5.14 Structures of(a) Fe 2 (CO)!-, (b) Fe,(CO)i;, (c) Fe.(CO)i; showing three doubly
170
bridging and one triply bridging CO groups (O atoms omitted).