Ligand vibrations

Metal isotope effects on metal-ligand vibrations. K. Nakamoto, Angew. Chem., Int. Ed. Engl., 1972,11,666-674(37). 

Metal-ligand vibrations have been identified using metal isotopes, e.g. the Ni—P stretching band which appears at 273.4 cm for NiCl2(PEt3)2 is shifted to 267.5 cm" for the Ni complex. 

Iron-Ligand Vibrations in Spin-Crossover Complexes... 

The vibrational spectrum of a metal complex is one of the most convenient and unambigious methods of characterization. However, it has not been possible to study the interactions of metal ions and biological polymers in this way since the number of vibrational bands from the polymer obscure the metal spectrum. The use of laser techniques for Raman spectroscopy now make it very likely that the Raman spectra of metals in the presence of large amounts of biological material will be measured (34). The intensity of Raman lines from metal-ligand vibrations can be... 

The possibility of obtaining this direct information has not been discussed previously in this context because vibrational fine structure is not generally observed in the ionizations of molecules of this size, and has never before been observed for any transition metal-metal vibrational mode. Our breakthrough in demonstrating that this fine structure can be observed has followed from several developments of our instrumentation. The details of these developments have recently been published along with our report of the first observations of metal-ligand vibrational fine structure in the ionizations of metal carbonyls QJ. 

Flint end Matthews (148) have used the AOM in a qualitative way in the analysis of the vibronic structure in the 4A2g -+ 2Eg absorption and 2Eg -> 4A2g luminiscence of octahedral Cr(III) chromophores. It can be predicted that ligand vibrational modes... 

Raman and IR of L2Fe2(CC 6, L = S ligand. Vibrational analysis and discussion of Raman intensities... 

Table I. Resonance-Enhanced Metal-Ligand and Ligand Vibrational Modes In Metalloprotelns ...

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

The absorption spectra of the lanthanide +3 cations are shown in Fig. 14.7. These spectra result from f-f transitions analogous to the d-d transitions of the transition metals. In contrast to the latter, however, the broadening effect of ligand vibrations is minimized because the 4/orbitals in the lanthanides are buned deep within the atom. Absorption spectra of the lanthanide cations are thus typically steip and line-like as opposed to the broad absorptions of the transition metals. 

The IR spectra of 15N-labelled complexes of A-p-tolylsalicylaldimines with zinc, copper and cobalt have yielded assignments of the metal-ligand stretching frequency and certain ligand vibrations.336 The v(M—N) values are metal-ion dependent in the order CoZn as expected from crystal field theory. Substituent-induced shifts are related to the residual polar effects of salicylaldimine substitution and to the inductive effects of N-aryl substitutents. 

The description of spin states as electronic isomers with different metal-ligand distances requires that their vibrational spectra be a superposition of the spectra of the two separate spin states. The relative contribution of the two states to the observed spectrum will change with temperature as the population of the spin states changes. This has been observed (76, 77, 122, 144, 145). Difficulties occur with the assignment of the metal-ligand vibrational frequencies of particular interest for the analysis of the dynamics of spin state transitions. Some success has been achieved with the use of metal isotopic labeling (82, 151), but there are few reliable assignments. 

In the far IR region three markedly metal independent bands are observed, Le. out-of-plane vibration 340cm-1, Eu mode vibration 300cm-1 and metal-ligand vibration 150-200cm-1.198 The first two bands also exist in the H2(Pc) spectra with the second split into two. 

The authors have interpreted their data using a statistical model, which reveals that the energy gap between the 1 Aj ground state and the lowest of the spin-orbitally split ST2 levels is temperature dependent the form of this temperature dependence changes with x. The model includes spin-orbit coupling, low-symmetry field distortion, covalency, and dynamic as well as static effects of local ligand vibrations. 

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