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

Transition metal chemistry
2Cr(COJ6 + 2e
-2CO
+2CO
2 [cr<coJ5r·--­
short life time
The metal-metal bonds in M 2 (C0) 10 (M = Mn, Te, Re) and in Co 2 (C0) 8 are
readily cleaved by sodium amalgam in THF:
1 Na/Hg 2e
,-Mn 2 (C0), 0 -------+ Mn(CO); -------+ Mn(CO)!-
THF ~
''JP~~ HMn(C0) 5
(pK ~ 7)
îCo 2 (CO). -------+ Co(CO); -------+ HCo(C0) 4 (pK ~ 2)
The resulting anions are the conjugate bases of carbonyl hydrides. HCo(C0) 4 is a
strong acid, but HMn(C0) 4 is much weaker. Some carbonyl hydrides such as
HCo(C0) 4 and H 2 Fe(C0) 4 are very unstable volatile materials, decomposing below
0°C. HMn(C0) 5 is somewhat more robust (dec. 80°C). It has a distorted
octahedral structure; the equatorial CO groups are bent towards the hydrogen. In
favourable circumstances hydrogen atoms attached to transition metals can now
be located quite precisely by X-ray diffraction in spite of their low scattering effect
on X-rays compared with heavy metal atoms. Neutron diffraction, when
available, provides a more accurate method, as scattering is effected by the nuclei,
and hydrogen presents a fairly high cross section. Spectroscopic methods, in
particular infrared and n.m.r. spectroscopy are invaluable in the initial characterization
of a transition metal hydride. Terminal M-H stretching vibrations
( 1700-2250 cm- 1 ) fall in about the same region as terminal carbonyl vibrations
(for HMn(C0) 5 , v(Mn-H) = 1783cm- 1 ). Confusion between the two can be
overcome by comparing the spectrum of the suspected hydride with that of the
corresponding deuteride when v(M-H)/v(M-D)"' fi. In their proton n.m.r.
spectra transition metal hydrides typically show resonances well to high field of
tetramethylsilane (- 7 to - 24<5; HMn(C0) 5 - 7.5<5). These shifts are very
characteristic of this class of compound. Most organic compounds resonate in the
region 10 to 0<5. The observation of coupling with metal nuclei with I = ~ such as
103 Rh, 195 Pt, 187 0s or 89 Y (p. 401) can provide evidence for direct M -H bonding
in specific cases.
Many carbonyl anions are strong nucleophiles. Some reactions of Mn(CO);
illustrate this (Fig. 5.15). They lead to various alkyl and aryl derivatives as well as
complexes in which Mn is bonded to Main Group and transition elements.
172