Preparation of the Metal Carbonyls

Susac et al. [33] showed that the cobalt-selenium (Co-Se) system prepared by sputtering and chemical methods was catalytically active toward the ORR in an acidic medium. Lee et al. [34] synthesized ternary non-noble selenides based on W and Co by the reaction of the metal carbonyls and elemental Se in xylenes. These W-Co-Se systems showed catalytic activity toward ORR in acidic media, albeit lower than with Pt/C and seemingly proceeding as a two-electron process. It was pointed out that non-noble metals too can serve as active sites for catalysis, in fact generating sufficient activity to be comparable to that of a noble metal, provided that electronic effects have been induced by the chalcogen modification. 

Synthesis of metal carbonyl clusters on oxide surfaces (followed by extraction into a solvent and workup) is occasionally a more convenient and efficient method for preparation of a metal carbonyl cluster than conventional solution chemistry. This synthetic strategy offers the green chemistry advantage of minimizing solvent use, as the reaction often occurs in the absence of solvent. 

Oxidation of the metal carbonyl is used in the preparations illustrated by equations (20) to (22),173-175 in the third of which a heteronuciear dimer is formed. No reaction occurred between Cr(CO)6 and 2-amino-6-methylpyridine.176 The binuclear nature of red [Cr PhC(NPh)2 2] has not been established, and an impure sample of [CrMo(mhp)4] has been prepared from [CrMo(02CMe)4].177... 

These complexes are related to those of Section 55.23.1, but have the added feature of Au Au bonding between the gold atoms. They are usually prepared from the metal carbonyl anion with [AuC1(PR3)] and some examples are given in Table 14. 

The photochemical synthesis of the tungsten carbonyl metallocarborane [Eq. (18) ] is also effective in the preparation of the molybdenum carbonyl analog. The resulting air-sensitive complexes show chemical behavior similar to the cyclopentadienyl analogs, C5H5M(CO)3, in that they undergo protonation with anhydrous HOI and met.hylation with CH3I. They also react further with metal hexacarbonyls to afford bimetallic complexes 54) ... 

The phosphinidenequadricyclane was converted to phosphinidenenorbornadiene by treatment with HgCl2, while its treatment with CuCl2 led to a phosphaallene derivative (equation 86)111,112. Weber and coworkers have prepared phosphaalkenyl metal complexes by reaction of the metal carbonyl complexes with the silyl phosphide 11a (equation 87)113. 

Although boron carbonyls are only stable at very low temperature, H3BCO reacts with hydroxide to give [H3BC02 2. which is stable at room temperature and is normally isolated as the mono sodium salt, Na[H3BC02H] [212], It was patented as a source of CO source for preparation of transition metal carbonyl complexes [213]. It is also a good source of CO for medical applications [214], It readily releases CO at physiological pH. 

It is possible to create more complex compounds and morphologies, such as alloys or core-shell structures. The nucleation step still occurs with injection of the metal carbonyl. However, when other organometallic precursors are in the solution, you can get the alloy to grow on the nuclei through transmetallation. To prepare core-shell material, the nuclei are allowed to react, then the temperature is reduced and a second precursor is introduced. The initial injection provided the nucleation which the additional materials when decomposed grow on the initial nuclei. 

The chemistry of the metal carbonyl hydrides and metal carbonylates remained the principal research topic for Hieber until the 1960s. He mentioned in his account [25], that it was a particular pleasure for him that in his laboratory the first hydrido carbonyl complexes of the manganese group, HMn(CO)5 and HRe(CO)5, were prepared by careful addition of concentrated phosphoric acid to solid samples of the sodium salts of the [M(CO)5] anions, giving the highly volatile hydrido derivatives in nearly quantitative yield [45, 46]. In contrast to HCo(CO)4 and its rhodium and iridium analogues, the pentacarbonyl hydrido compounds of manganese and rhenium are thermally remarkably stable, and in... 

Although a variety of new preparative routes has been developed in recent years (for reviews see refs 1 -10), the transformation of the metal-carbonyl carbon bond of a metal-carbonyl complex into a metal-carbene carbon bond is still the most useful and versatile method for preparing transition-metal carbene complexes. The addition of a carbanion to the carbon atom of a carbonyl ligand yields an anionic acyl complex that subsequently can be reacted with an electrophile to give a neutral carbene complex. Thus, the syntheses of anionic acyl and neutral carbene complexes are closely related, for almost all the carbene complexes considered in this section acyl complexes are precursors, although most have not been isolated and characterized. The syntheses of acyl complexes via CO insertion (for reviews see refs. 11, 12) or by reaction of metal carbonyl anions with acyl halides is outside the scope of this section. 

Electrochemical preparation of several metal carbonyls of V, Cr, Mn, Fe, Co, Ni, etc., has been achieved by using an electrode made of an electropositive, readily dissolving metal, able to form the carbonyl at the cathode (Al) in the presence of CO. Tetraalkylammonium salts provide conductivity in such organic media as py. Anodic reactions have also been used . 

A method which has been used mainly for the preparation of cyclopentadienyl metal carbonyls, but which is occasionally applicable, is that represented rouglily by the equation... 

E22.5 As discussed in Section 22.18, the two principal methods for the preparation of simple metal carbonyls are (1) direct combination of CO with a finely divided metal and (2) reduction of a metal salt under CO pressure with a presence of a reducing agent. Two examples are shown below, the preparation of hexacarbonylmolybdenum(0) and octacarbonyidicobalt(O). Other examples are given in the text. 

The [M2(CO)io(p.-H)] derivatives have been the subject of several recent structural studies.6 X-ray and infrared studies on its tetraethylammonium salt, [Cr2(CO),0(p,-H)] , first indicated this to be the first example of a species containing a linear electron-deficient M-H-M system. This was later found to be nonlinear by neutron diffraction studies.7 The chemical reactivity of these complex species is currently being studied, because they are of interest both for their applications in catalysis8 and for the preparation of other metal carbonyls. The [Cr2(CO)io(p.-H)K complex, for example, is easily converted into [Cr2(CO),0(p.-X)] or into [Cr(CO)5X] , thus providing a simple and rapid method for a high-yield synthesis of these species.3... 

Sample Preparation. Cobalt catalysts were prepared by subliming Co2(C0)g into the pores of dehydrated NaX zeolite in a vacuum line at pressures of 1 x 10- f torr. Argon was flowed over the metal loaded zeolite sample at a pressure of 0.3 torr. A microwave plasma was induced with a static gun and the decomposition of the metal carbonyl precursor occurred for two hours. After total decomposition of the metal carbonyl which can be determined by the color of the plasma, the argon flow was stopped and the sample was sealed off by closing the Teflon stopcocks at both ends of the reactor. The sample was then brought into a drybox and loaded into catalytic reactors or holders for spectroscopic experiments. Further details of this procedure can be found elsewhere (11, 25). Iron samples were prepared in a similar fashion except ferrocene was used as a metal precursor. 

Recent examples of preparative applications of photosubstitution reactions of metal carbonyls are collected in Table 5 (see p. 193). Others, less suited to tabular presentation, are discussed below in the section dealing with the particular element. In general, synthetic methods involving initial photochemical formation of the metal carbonyl-THF complex, followed by thermal reaction of this complex with an added substituent, have not been included. 

Nearly all of the compounds were synthesized photochemically, which imposes a nuihber of quite strict limitations on larger-scale preparations of these compounds. The primary, or at least the initial, identification of these compounds has been via IR spectroscopy. (The sole exception has been Cp Re(CO)(C2H4)2 which was tentatively identified by NMR [18]). In most cases, characterization of the metal carbonyl moiety via v(C-O) IR bands is definitive, particularly if library spectra are available from liquid noble gas [23] or matrix isolation experiments [4]. This is because these bands are sharp and intense, and the C-O stretching vibrations are largely uncoupled from other vibrations of the molecule. Furthermore, the precise wavenumber of the bands are extremely sensitive to the oxidation state of the metal center as illustrated, for example, by Kazarian et al. in their study of hydrogen bonding to metal centers [25]. 

One of the major factors leading to the explosion, during the 1950s, in the chemistry of the metal carbonyls and other cluster compounds was the availability of instrumentation and experimental techniques to deal with these compounds. Many of the preparative techniques used in the formation of these compounds are often not difficult or sophisticated. The main problem was the identification of the compounds once they had been prepared. Even the basic analytical data could often be ambiguous when dealing with high molecular weight compounds, and the determi-... 

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