Preparation of the Hexacarbonyls M CO

Sulfoxide adducts of chromium, molybdenum, and tungsten carbonyls have been studied as catalysts for the polymerization of monomers such as vinyl chloride (248). Simple adducts of the type [M(CO)5(Me2SO)] may be prepared by carbonyl displacement from the corresponding hexacarbonyl. Photochemical reactions are frequently necessary to cause carbonyl displacement in this manner, many carbonyl complexes of higher sulfoxides have been prepared (255, 256). Infrared (257) and mass spectral studies (154) of these complexes have appeared, and infrared data suggest that S-bonding may occur in Cr(0) sulfoxide complexes, although definitive studies have not been reported. 

The majority of these contain CO ligands. The neutral homoleptic carbonyls have not been isolated, but anionic M(CO)6 as well as highly reduced M(CO) species are known. The original synthesis of the -1 species required elevated temperatures and high pressure but recently two simple, atmospheric pressure methods have been developed. They involve reduction of pentahalides in pyridine with Zn/Mg or in dimethoxyethane with sodium naphthalenide under an atmosphere of CO. These yellow salts contain discrete M(CO)6 anions. The facile syntheses of the octahedral hexacarbonyl anions allowed systematic exploration of the previously difficult to access area of low-valent complexes of Nb and Ta. Since M(CO)s are rather inert towards displacement of CO the substitution products of general formula M(CO)6- L have to be obtained by other routes, for example, by reduction of MX(CO)6- L compounds. The monosubstituted derivatives are conveniently prepared via the following method87 ... 

Potassium salts of pyrrole, 2-acetylpyrrole, 1,2,3,4-tetrahydrocarbazole, 9-carbazole, indole, imidazole, and 1,2,4-triazole react with [Tr-CjHjFe ( 0)2 ] to form ct-N derivatives, e.g., [7r-C5H5Fe(CO)2(CT-N-pyrrolyl)] (III) (400, 421), shown to be intermediates in the formation of the n-complexes e.g., (IV) (400). Anions [M(CO)5L] (M = Cr, Mo, W) were similarly prepared from the hexacarbonyls and alkali metal derivatives of succinimide, phthalimidine, and saccharin (49). 

Since two CO groups of the hexacarbonyl are replaced to form the [M(CO)4]" anions, these highly reduced anions are prepared" by reducing diamino-substituted tetracarbonyl derivatives with sodium in liquid NH3 ... 

Thermal displacement of carbonyl ligands has rarely been used to prepare olefin complexes of group-VIA metals, although Mo(CO)s and acrylonitrile form the diamagnetic, insoluble and probably polymeric Mo(CH2=CHCH)2(CO)2 Where the group-VIA hexacarbonyl is to be refluxed with an olefin problems arise because the unreacted M(CO)s sublimes out of the reaction flask. This may be partially alleviated using special... 

The most intensely studied species are the group 6 pentacarbonyls, M(CO)5(H2), which have been observed in rare-gas matrices, in liquid Xe solutions at - 70°C (a very useful medium), in alkane solvents, and in the gas phase. Perhaps the most novel preparation is photolysis of the hexacarbonyls impregnated in polyethylene disks under H2 or N2 pressures to give M(CO)6 n(L)n, where n = 1,2 for L = H2 and 1-4 for N2.175 Reactivity follows the order Mo > Cr > W, and H2can displace coordinated N2 in the polyethylene systems. In all media, vibrational spectroscopy provides evidence for H2 rather than dihydride binding, and the H-H, H-D, and... 

Group VIb elements provide by far the most representatives of carbonyl complexes of cyclophanes. Their preparation is achieved by heating the metal hexacarbonyl M(CO)g (M = Cr, Mo, W) with the respecting cyclophane in a high boiling point solvent, such as ligroin or di-n-butylether, for several hours... 

Typically the desired isocyanide substitution products are obtained in good yield from such reactions. However, a major drawback of this strategy entails the prior preparation and purification of the labile precursor complexes. The transition metal catalyzed substitution of the metal hexacarbonyls [M(CO)e] (M = Cr, Mo, W) by isocyanides has, by way of contrast, proved to be a direct and rapid synthetic route that reliably gives the products [M(C0)6, (CNR)J (n = 1-3) in high yield. We describe here general procedures that illustrate the methods involved and that highlight the synthetic utility of these reactions. 

The hexacarbonylniobate(—1) and hexacarbonyltanta-late(—1) anions are well established. The [Nb(CO)6] anion can be prepared at atmospheric pressure of CO from NbCls by using a prereduction step with alkali metal/naphthalene at low temperature, followed by carbonylation of the intermediate at low temperature in DME as solvent. Another method of preparation, also operating at atmospheric pressure of CO, uses the reduction of NbCls with Mg/Zn/pyridine at room temperature and gives yields as high as 48% of recrystallized Na(THF)[Nb(CO)6]. The alkali metal derivatives of [M(CO)6] , M = Nb, Ta, are rapidly oxidized by air the PPN derivatives are stable in air for short periods of time. PPN[Nb(CO)6] and PPN[Ta(CO)6] are isostructural the hexacarbonyl anions possess an almost exact octahedral geometry with the following bond distances Nb-CO, 2.089A Ta-CO, 2.083 A. 

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