Metal binding, infrared spectroscopy

Cyanide is also an effective infrared probe (Yoshikawa et al., 1985). A drawback of this reagent as an infrared probe is its infrared intensity, which is much weaker than that of CO. However, as given in Fig. 10, the recent development in the infrared technique has solved this problem with the introduction of a mercury/cadmium/tellurium (MCT) detector (Fig. 10) (Yoshikawa et al., 1995). The C-N stretch vibrational band is sensitive to many factors, such as the oxidation state and species of the coordinating metal, the structures of porphyrin ring substituents, and the ligand trans to the cyanide and protein structure (Yoshikawa et al., 1985). This technique can be quite effectively applied for determination of the protonation state of the cyanide bound at a metal site. Possible binding modes of cyanide to a ferric iron are shown by Structures (1), (11), and (HI). Infrared spectroscopy is the best method for identihcation of these... 

Catalysis by Supported Metal-cluster Compounds. Further work has been reported recently on methods of chemically binding cluster compounds to supports and on the characterization of the resulting materials by various spectroscopic techniques. For example, the reaction of Rh6(CO)i6 with amine- and phosphine-modified silicas has been examined by infrared spectroscopy and has shown that cluster breakdown occurs giving L Rh(CO)2 and Lfn I (CO), where L comprizes the surface attached ligands. This behaviour is similar to that observed with Rh4(CO)i2 on unmodified silica where cluster breakdown occurs readily, particularly in the presence of traces of water and/or oxygen. ... 

Peptides are known to show some specificity toward metal binding, as was shown by titration experiments nsing H-NMR [7,8], potentiometry [9], luminescence measnrements [10], infrared (IR), circular dichroism (CD), and ultraviolet (UV spectroscopies [11]. Poly-L-aspartic acid binds to metals such as Eu, Ce, La +, Cu ", and Pb ", and acts as a corrosion inhibitor for steel and iron. This property has been ascribed to the carboxylate side chain of aspartic acid [12,13]. 

Infrared spectroscopy of olefin complexes is a less useful probe of n-bonding than infrared spectroscopy of CO complexes. Binding of an olefin to an electron-rich metal center does reduce the C-C stretching frequency, as one would expect from the reduction of the C-C bond order due to Ti-backbonding. However, the C-C stretch of a coordinated olefin is weaker than that of coordinated carbon monoxide because the vibration of the olefin creates a smaller change in the dipole moment. (Recall that symmetric vibrations are not observed in the infrared spectrum because of a lack of change in the dipole moment.) Thus, the olefin stretch is weak and lies at a frequency that overlaps with other bands. 

The effect of metal-ion concentration on the stoichiometry of surface complexes was studied by in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) spectroscopy, monitoring the binding of Cd + to a carboxylate-terminated SAM [29]. Specific... 

Infrared spectroscopy is especially useful for metal carbonyls because the intense C=0 stretching vibration at 1700-2100 cm appears a region that is relatively free of other bands. The intensity is large thanks to the polarization on binding (M-C +=0 ) and consequent large d ildr. In polycarbonyls, the v(CO) bands are coupled in a way that depends on the symmetry of the M(CO) fragment. 

---