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

Transition metal carbonyls
Table 5.4 Band positions in the vibrational spectra of complexes
Ni(CO)"L 4 _", L = P(OMe),
Ni(C0) 4 Ni(CO),L Ni(C0) 1 L 1 Ni(CO)L,
Point
Group Td c3\ el, e"
v(CO)
v(MC)
Al 2131 A, 2082 A 1 2025 Al 1964
T 2 2058 E 2006 B 1 1965
2015
Al 368 Al 420 Al 458 Al 499
Tl 423 E 455 Bl 495
The bands labelled A 1 arise from the symmetric breathing modes. For
Ni(C0) 4 • this mode is Raman, but not infrared active (change in
polarizability but not in dipole moment). The bands Iabelled T 1 are due
to the antisymmetric modes which are infrared. but not Raman active.
For C,v• C 1 v ali modes shown are both Raman and infrared active.
example. the P-Et bond decreases from 1.~46( 3) Ă to 1.~29( 3) Ă. while the
Co-P distances increase from 2.218(l)Ă to 2.230(l)Ă.
This change in interpretation does not affect the general picture of a phosphine
as a rr-donor and a n-acceptor ligand. The rr-donor power has been related to the
pKa values (p. 12 7). Trialkylphosphines therefore are quite strong rr-donors but
trialkylphosphites are rather weak. The rr-donor ability increases across the
series, PF, < P(OMe), < PPh, < PMe,. It is much more difficult to quantify the
n-acidities. It is generally agreed. however. that the weakest rJ-donors are
the strongest rr-acceptors and vice versa. Some people suggest that trialkylphosphines
such as PBu, may lack allrr-acceptor ability and may cven ben donors.
Some band positions in the vibrational spectra of the complexes
Ni(CO)"L 4 " (L = P(OMe),) are given in Table '5.4. On succcssivc substitution
of CO by P(OMe), the position of v(CO) falls. indicating increased back
donation into the rr* orbitals of CO. This is consistent with the proposal that
P(OMe), is a stronger rr-donor than CO but a weaker rr-acceptor. The trend in
v(MC) is consistent with this suggestion.
Tolman correlated the position of the A 1 band in the infrared spectra of
complexes Ni(CO),PR, in dichloromethane with the electronic effect of the
substituents R (Table 5. 5 ). A reasonable correlation with Hammett rJ-parameters
was also found. The ligand behaviour ofphosphorus donors are thus defined both
by these electronic effects as well as by the steric effects measured by the cone
angle (p. 169).
Where a molecule is fairly symmetrical different structures can be distinguished
using vibrational spectroscopy. This method has been used, for example. to
characterize geometrica! isomers of substituted octahedral complexes. The point
groups and symmetry classes of the normal v(CO) vibrations for octahedral
complexes M(CO)"L 6 " are listed in Table 5.6. Sometimes infrared spectroscopy
lh 1