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

Transition metal chemistry
Table 5.5. Band positions of the A1 stretching mode for the complexes
Ni(CO),PR,
v(CO)
R (cm- 1)
C(CH,), 2056.1 Best CJ donor Worst n acceptor
Me 2063.9
Ph 2068.9
OMe 2079.2
OPh 2085.3
l j
F 2110.7 Worst CJ donor; best n-acceptor
Table 5.6. Infrared and Raman active carbonyl stretching modes for octahedral complexes
M(CO)nL 6 _n
No. of
Point IR Ram an No. of Raman
Molecule group active active IR bands bands
M(C0) 6 oh T1u A1g + Eg l 2
LM(CO), c.v 2A 1 +E 2A1 + B 1 +E 3 4
cis-L 2 M(CO). c2V 2A 1 +B 1 + B2 2A 1 +B 1 +B2 4 4
trans-L2M(C0) 4 D.h Eu Alg + Blg l 2
mer-L,M(CO), D2h 2A1 + B 1 2A 1 + B 1 3 3
fac-L,M(CO), c,v A 1 +E A 1 +E 2 2
cis-L,M(C0) 2 c2V Al+ Bl Al +Bl 2 2
trans-L.M(C0) 2 D.h A2U Alg 1 1
L,M(CO) c.v Al Al l l
alone is sufficient to distinguish isomers. Thus cis-L 2 M(C0) 4 affords faur infrared
active bands, whereas trans-L 2 M(C0) 4 shows only one. In cases of doubt,
however, it is always wise to check using Raman spectroscopy. Ifthe ligands L are
unsymmetrical. the symmetry of the molecule will depart to some extent from the
ideal. This can lead to the observation ofweak bands which would be excluded in
the pure symmetry class. Moreover the degeneracy of vibrational modes can be
lifted; doubly degenerate bands (E), for example, often split into two under these
circumstances. The concept of 'local symmetry' within a molecule can also be
useful. The pyramidal M(C0)3 unit, which belongs to the point group C3v gives rise
to 3 v( CO) fundamentals which are both infrared and Raman active (A 1 + E). This
pattern, possibly with the E band split into two, appears in the spectra of ali the
complexes C 7 H 7 V(C0) 3, C 6 H 6 Cr(C0)3, C 5 H 5 Mn(C0)3, C 4 H 4 Fe(C0)3 as well asfac­
L3M(C0)3.
5.2.4 Mass spectra
Electron impact mass spectra of carbonyl complexes commonly show successive
stepwise loss of all the carbonyl groups from the molecular ion (Fig. 5.11). The
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