Spectroscopic Investigation of Some Metal VI Carbonyl Phosphine Complexes

Preparation of metal carbonyl phosphine complexes of type LM(C0)5 and trans L2(CO)4 M = Cr, Mo and W; L = P(OMe)3, P(OEt)3, P(OPri)3, PPri3 and PEun3 has been studied in a systematic way. Direct solventless reaction has been found applicable for preparation of the trans disubstituted derivatives, especially those of tungsten which are difficult to obtain in other ways. Chromatography was used for separation and purification of the complexes. The ultraviolet spectra of the phosphine complexes have been recorded both at room temperature and. low temperature. The low temperature spectra were obtained using a sample cooled by liquid nitrogen in a dewar. The observed change in the spectral outline on sample cooling has been successfully related to the forbidden/allowed character of the electronic transition involved. Using this experimental evidence, the spectra of the metal complexes have been partially assigned for the energies approximately 35,000 - 25,000 cm-1 where transition energies may represent either d-d or charge transfer type transitions and the extinction coefficients at room temperature have values intermediate between accepted d-d and charge transfer values. Thus the lowest energy charge transfer transition has been assigned for thirty complexes, in only three of which it has been previously reported and these on the basis of spectra recorded at room temperature only. Some representative examples of the ultraviolet spectra having several overlapped, partially resolved bands were analysed using a combination of Gaussian curves, This technique has become more popular recently and has been applied to complex spectra of derivatives of the type studied here. The spectral outlines were not all suitable for curve analysis since in cases where the degree of overlap is very high, it is not possible to make a plausible assessment of the number of peaks involved, this information being essential for the analysis. Comparative critical studies have been carried out on the Gaussian analyses obtained, taking account of the subjective factors of the operator's decisions concerning the number of peaks and their relative intensities. This has been used in assessing both the validity and usefulness of the peak information revealed by such analyses. Nuclear magnetic resonance spectra of the complexes have been recorded for proton and carbon-13 resonance. The proton spectra of the trans disubstituted phosphite derivatives have been related to the phenomenon of virtual coupling which is seen for protons on the alpha-carbon of the ligand substituent only. Analogous phosphine derivatives are seen to have second order splitting patterns which would require further analysis, probably by computer simulation, to be discussed in relation to the phosphite spectra. The carbon-13 spectral parameters are reported for these complexes for the first time. Chemical shifts of the carbon in the carbonyl groups and in the ligands along with coupling constants for phosphorus-carbon and tungsten-carbon coupling are given. Their discussion is limited by the undeveloped nature of chemical shift and coupling theory as they apply to carbon-13 magnetic resonance. The Mass spectra of a limited number of the complexes are reported and a short discussion of the possible fragmentation patterns is given. These patterns are derived by analogy with previously reported patterns of similar complexes. The spectra of two monosubstituted complexes are compared with those of the corresponding trans disubstituted derivatives and the differences in relative intensities and m/e fragments observed are discussed. Infrared spectra of the complexes have been recorded in the stretching carbonyl region on solutions of very high concentration and formally forbidden peaks have been observed and assigned for mono-and trans disubstituted phosphite derivatives while only for the monosubstituted phosphine derivatives have such peaks been assigned. The deltaMCO and nuMC absorptions have been recorded although, as in previous studies, fewer peaks are observed than are required by group theory. Metal phosphorus bonding is discussed taking account of the U.V., I.R. and N.M.R. studies reported here along with recent developments in relevant molecular orbital theory and other reported results concerning bonding in similar complexes. An assessment has been made of the value of relating spectroscopic results directly to variations in the metal-ligand bonds, especially the metal-phosphorus bond. A short appendix deals with preparation of trifluorophosphine derivatives of M2(CO)10M =Mn, Re