The assignments of the peaks in regions B and C are central to the resonance Raman analysis. The signal in region C in [R(en)Cl2][R(en)Cl4] is at ca. 210 cm’^ and shows little
dispersion or enhancement when the excitation energy is altered; similar behaviour is seen for [Pt(en)Br2][Pt(en)Br4]. This is in complete contrast to the vim mode, which dominates this
region in the spectra of the mixed-halides, 312a-312d. The complementary V2m signal, which
could confirm the presence of vim in [R(en)Cl2][R(en)Cl4], is not observed clearly in any of the
HMMCs because it falls within the contour of the v(PtCI) vibrational modes. Instead, the peaks in region C for [R(en)X2][Pt(en)X4] (X = Cl or Br) are usually attributed to the S(RN2) bending
mode because it is fairly independent of the halogen identity.®®-®® This is the most reasonable assignment that can be derived from a conventional model of the HMMC system. Altemative explanations are hard to prove or involve too many assumptions. For instance, the termini of chains will give rise to defect modes, but the way in which chains terminate has never been addressed, probably because such sites will make only a small contribution to the vibrational spectra. In theory, the terminal atom can be R", R^^ or X(-Pt'^). If it is platinum, then the identity of the neighbouring halogen might not have much influence on the vibrational energy.
The assignment of for the signals in region B can be discounted for the following reasons. There is no intensity in region B of the single-crystal spectra of (Pt(en)Cl2][Pt(en)Cl4],
the excitation energy dependence of the signal in the disc spectra is different to that of v^b in the brominated HMMCs, and there are no peaks assigned either to 2vib or to vi^. The peak in region I has a different relationship to excitation energy from that of 2vib, and it is present in single-crystal spectra that have no intensity in region B, and so it is attributed to the weakly Raman-active asymmetric vibration, V2c. The lack of a signal is significant because
mixed-halide units are produced in the reaction between Br and [Pt(en)2Cl2]^* ions,^^ and so
there should be some correlation between the intensities of v^b and of v^m- These results mean that certain observations that appear to support the assignment of the signal in region B to the VIb mode can be ignored. For instance, the peak itself has a dispersion similar to that shown by vib in the spectra of [Pt(en)Br2][R(en)Br4], and the spectrum of [Pt(en)Cl2][Pt(en)Cl4]
the Vim signal occurs in the mixed-halide species. There are altemative explanations for the peak region B. The bending mode 5(PtCl2) has been pr oposed, but weak signals have been found in this region for complexes that have no equatorial halogen atoms (see section 4.4). In addition, there is little or no intensity in this region in single-crystal spectra, and the related mode ô(PtBr2) has not been assigned. Of the more conventional vibrations, V3c is a suitable
candidate. A suggested assignment for it In (Pt(en)2][Pt(en)2Cl2](CI0 4 ) 4 is ca. 165
although this has not been confirmed, v^ should have little Raman intensity, particularly in more uniform crystal structures, but the presence of terminations might reduce the asymmetry of the mode and increase its Raman activity. Other possibilities are connected with the terminal modes that were discussed in relation to the vibration in region C. Support for them is derived from comparisons of the spectra for samples analysed as pressed discs and as single-crystals. The greater intensity in the B region of the disc spectra may be due to the larger number of chain fractures in the discs.
Infrared spectroscopy is often a source of results complementary to Raman studies, but the FT-infrared spectra of the neutral-chain HMMCs do not contribute any significant data to this work. They were recorded at room temperature and so they are not well resolved. Selected spectra are displayed in Figure 3.3.10. The peaks pertinent to this discussion are those due to the modes V2b, V2m and vim. V2b is probably the large signal at ca. 225 cm'^ in the
spectrum of 312d; approximate wavenumbers are known for V2m and vim from their Raman
signals. Unfortunately, there are ligand modes and equatorial modes that obscure the regions of the spectrum in which vi^ or V2m appear, and it is difficult to tell whether there is any
intensity at ca. 225 cm'^ for [Pt(en)Cl2][Pt(en)Cl4].
All assignments for the mixed-valence species must be tempered by the knowledge that [Pt(en)Cl2][R(en)Cl4] has been doped imperfectly with HBr. From the solid-state NMR
results it has been shown that up to about 40 % of the bromine ions added exchange with the equatorial chlorine atoms. None of the axial modes is expected to be affected significantly by this and only one extra mode is thought to occur as a result of equatorial substitution, namely the weak signal in region E. The purpose of the bromination experiment was to determine whether any of the peaks in the Raman spectra of [Pt(en)Cl2][Pt(en)Cl4] are due to bromide
impurities. The synthesis of [Pt(en)Cl2][Pt(en)Cl4] does not involve directly any bromine or
bromide ions, but the ease with which heavy halogens replace light ones in HMMCs means that contamination can result from a very small amount of impurity. Bromine atoms are known to replace bridging chlorine atoms in HMMCs readily in solution or even when the solid linear-chain is ground with a bromide salt.^"^ The starting materials (Pt(en)Cl2 or en.2HCI) are
possible sources of contamination. However, the results in this section show that the weak signals in the spectra of [Pt(en)Cl2][Pt(en)Cl4] are not consistent with or vib, and so the
chloride samples are assumed to be uncontaminated.
(a)[Pt(en)a,]pt(en)a, (b) Complex 312a
W ^
'(c) Complex 312c (d) Complex 312d
m
400 \AAvenumber/cm'^