Transition metal complexes infrared spectra

The band at 1600 cm-1 due to a double-bond stretch shows that chemisorbed ethylene is olefinic C—H stretching bands above 3000 cm-1 support this view. Interaction of an olefin with a surface with appreciable heat suggests 7r-bonding is involved. Powell and Sheppard (4-1) have noted that the spectrum of olefins in 7r-bonded transition metal complexes appears to involve fundamentals similar to those of the free olefin. Two striking differences occur. First, infrared forbidden bands for the free olefin become allowed for the lower symmetry complex second, the fundamentals of ethylene corresponding to v and v% shift much more than the other fundamentals. In Table III we compare the fundamentals observed for liquid ethylene (42) and a 7r-complex (43) to those observed for chemisorbed ethylene. Two points are clear from Table III. First, bands forbidden in the IR for gaseous ethylene are observed for chemisorbed ethyl-... 

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible and, for many transition metal complexes, the near infrared region of the electromagnetic spectrum. Spectroscopic methods that probe these electronic transitions can, in favourable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

There followed many other exciting adventures involving studies of the infrared spectra of transition-metal complexes, but I will mention just one which I find particularly memorable. This followed the discovery by Bernard Shaw of a remarkably stable volatile platinum complex produced by reduction of [PtCl2(PEt3)2], the spectrum of which had a very strong and sharp absorption band near 2200 cm ... 

The structure assigned to (XXVII) is primarily based on its infrared spectrum and that of the analogous deuteride. Such hydrido complexes are probably intermediates in the pol rmerization of terminal acetylenes by transition metal complexes 91). In this regard it is significant that phenyl-acetylene forms an oligomer in the presence of a catalytic amount of the rhodium complex RhCl(CO)(PPh3)2 32). Rhodium remains chemically bound to this poorly characterized oligomer. 

It must be emphasized, however, that the frequencies listed in various inorganic structure-spectra correlation charts are for the ionic salts. There are many instances in which the polyatomic anion (c.., NO3, SO4 , col , IO3) becomes coordinated to the cation, and this causes a lowering of the symmetry of the anion. These are anions that have double or triple degenerate vibrations. This is reflected in the infrared spectrum by additional absorptions because of the removal of the degeneracy. Many examples of this are illustrated in the transition-metal complexes involving the above anions [ ]. 

An absorption (1030 nun) found in the near-infrared spectrum of this complex arises from a mixed valence transition. Light-induced meial-to-metal charge transfer was predicted by Hush56 for systems of this type before it was observed experimentally. Further, his theory relates the energy of absorption to that required for thenral electron transfer (hv = 4 x Ec) and from this it is possible to calculate the thermal electron transfer rale constant (5 x (08 s-1).57... 

The transition-metal allyl complexes are air- and temperature-sensitive solids Cr(allyl)3, m.p. ca. 70° Ni(allyl)2, m.p. ca. 0°. The infrared spectrum of both compounds indicates that the bonding of the allyl group to the metal involves r electrons (the olefinic bond appearing at 1520 and 1493 cm.-1, respectively) they can be identified by their mass spectra. 

Formula II shows one trans-double bond which is shared with the nickel atom. Furthermore, there are six carbon atoms which are in the state of an -hybridization. Each C atom shares one r-electron with the nickel. (The complex shows the correct molecular weight for NiCi2Hig, and there is no absorption in the infrared spectrum characteristic of double bonds.) This formulation has some relationship to structures which have been recently proposed by different authors (2, 4) for various allylic groups bonded to transition metal carbonyls. 

The ultraviolet spectrum of quinoxaline 1-oxide in a Nujol mull shows 238, 252 sh, 325, 340 sh, and 348 nm. Both ultraviolet and infrared data have been used to assign structures to the complexes quinoxaline 1-oxide forms with transition metal chlorides. 

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