Absorption spectra metal-carbonyl complexes

The intermediate cyclooctene complex appears to be more reactive with respect to CS coordination and more sensitive to oxidation when the arene ring bears electron-withdrawing groups (e.g., C02CH3). Dicarbonyl(methyl rj6-benzoate)-thiocarbonyl)chromium is air stable in the solid state and reasonably stable in solution.9 The infrared spectrum exhibits metal carbonyl absorptions at 1980 and 1935 cm"1 and a metal thiocarbonyl stretch at 1215 cm"1 (Nujol) (these occur at 1978, 1932, and 1912 cm"1 in CH2C12 solution).10 Irradiation of the compound in the presence of phosphite or phosphine leads to slow substitution of CO by these ligands, whereas the CS ligand remains inert to substitution. The crystal structure has been published."... 

The increase in A(J caused by tt bonding is substantial enough in many cases that the absorption maximum for the h -to-ejj electronic transition is blue-shifted out of the visible region into the ultraviolet portion of the electromagnetic spectrum, with the result that the complexes are colorless. This is the case for the metal carbonyls, for example. 

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. 

Recrystallization from pentane gave attractive yellow crystals. The structure of 2 could be determined by X-ray diffraction analysis (Figure 2). The NMR spectroscopic results were consistent with the assigned structure (Table 1). The IR spectrum shows three v(CO absorptions at 1965, 1991 and 2044 cm. This behavior is typical for metal carbonylates of the type c/s-L2M(CO)3 Compound 2 is more stable than the previosly described (ri -C5H5)M(CO)3SiMe2SiMe2Br (M=W, Mo) [5], Complex 2 decomposes thermally below 150°C and is stable against light. Decomposition under the influence of donor solvents was not observed. 

Another large family of d complexes with MLCT absorptions which, however, appear in the UV region comprises cyano complexes [1] (e.g. [M(CN)6]4- with M = Fe, Ru and Os and metal carbonyls [1,4,43] e.g. M(C0)6 with M = Cr, Mo, W). These MLCT bands may obscure less intense bands of different origin such as LF absorption. A characteristic long-wavelength MLCT band was also identified in the abs. spectrum of the carbene complex Mn(C5H5)(CO)2(CPh2) (X iax = 380 nm. Fig. 5) [44]. 

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