Infrared and Raman Spectra of Inorganic and Coordination Compounds: Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry, Sixth Edition

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the next subsection, a simple 1 : 1 complex such as that shown in Fig. 1.2 has been
prepared in inert gas matrices [30].
To assign the skeletal modes such as the MN stretching and NMN bending modes, it
is necessary to consider the normal modes of the octahedral MN6 skeleton (Oh
symmetry). The MN stretching mode in the low-frequency region is of particular
interest since it provides direct information about the structure of the MN skeleton and
the strength of the M
N bond. The octahedral MN6 skeleton exhibits two n(M
N)
(A1g and Eg) in Raman and one n(M
N) (F1u) in infrared spectra (see Sec. 2.8 of
Part A). Most of these vibrations have been assigned on the basis of observed isotope
shifts (including metal isotopes, NH3/ND3 and NH3=15NH3) and normal coordinate
calculations. Although the assignment of the n(Co
N) in the infrared spectrum of [Co
(NH3)6]Cl3 had been controversial, Schmidt andM€uller [4] confirmedthe assignments
that the three weak bands at 498, 477, and 449 cm
1 are the split components of the
triply degenerate F1u mode (Fig. 1.4). The intensity of the MN stretching mode in the
infrared increases as the M
N bond becomes more ionic and as the MN stretching
frequency becomes lower. Relative to the Co(III)
N bond of the [Co(NH3)6]
3þ ion,
the Co(II)
N bond of the [Co(NH3)6]
2þ ion is more ionic, and its stretching frequency
is much lower (325 cm
1
). This may be responsible for the strong appearance of the
Co(II)
N stretching band in the infrared. As listed in Table 1.1, two Raman-active
MN stretching modes (A1g andEg) are observed for the octahedral hexammine salts. In
general, n(A1g) is higher than n(Eg). However, the relative position of n(F1u) with
respect to these two vibrations changes from one compound to another. Another
obvious trend in n(MN) is n(M4þ
N) >n(M3þ
N) > n(M2þ
N). This holds for all
symmetry species. Table 1.1 shows that the NH3 rocking frequency also follows the
same trend as above.
Normal coordinate analyses on metal ammine complexes have been carried out by
many investigators. Among them, Nakagawa, Shimanouchi, and coworkers [13] have
undertaken the most comprehensive study, using the UBF (Urey–Bradley Force) field.
The MN stretching force constants of the hexammine complexes follow this order:
PtðIVÞ  CoðIIIÞ > CrðIIIÞ > NiðIIÞ  CoðIIÞ
2:13 1:05 0:94 0:34 0:33 mdyn=A
Acevedo and coworkers carried out normal coordinate calculations on the [Cr
(NH3)6]
3þ and [Ni(NH3)6]
2þ ions [14,15]. On the other hand, Schmidt and M€uller
[4,5] and other workers [8] calculated the GVF (generalized valence Force) constants
of a number of ammine complexes by using the point mass model (where the NH3
ligand is regarded as a single atom having the mass of NH3), and refined their values
with isotope shift data (H/D, 14N=15N, and metal isotopes). For the hexammine series,
they obtained the following order:
Pt4þ > Ir3þ >Os3þ > Rh3þ >Ru3þ > Co3þ >
2:75 2:28 2:13 2:10 2:01 1:86
Cr3þ > Ni2þ >Co2þ >Fe2þ Cd2þ >Zn2þ >Mn2þ
1:66 0:85 0:80 0:73 0:69 0:67 mdyn=A
4 APPLICATIONS IN COORDINATION CHEMISTRY

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