Mononuclear carbonylation

Mononuclear Carbonyls. The lowest coordination number adopted by an isolable metal carbonyl is four. The only representative of this class is nickel carbonyl [13463-39-3] the first metal carbonyl isolated (15). The molecule possesses tetrahedral geometry as shown in stmcture (1). A few transient four-coordinate carbonyls, such as Fe(CO)4, have also been detected (16). 

Flaving the d s configuration, the elements of this triad are able to conform with the 18-electron rule by forming mononuclear carbonyls of the type M(C0)5. These are volatile liquids which can be prepared by the direct action of CO on the powdered metal (Fe and Ru) or by the action of... 

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

The simplest transition metal carbonyls are mononuclear of the type M(CO)x, in other words those with only one metal atom. They are hydrophobic but soluble to some extent in nonpolar liquids, such as n-butane or propane. The dinuclear carbonyls are more complex but have the same general characteristics as the mononuclear carbonyls. The carbonyls, which are or could be used in CVD, are listed in Table 3.4 with some of their properties. 

FIGURE 21.1 The structures of the mononuclear carbonyls of nickel, iron, and chromium. 

In none of the cases discussed above is molecular hydrogen involved. The first report of the stoichiometric reduction of coordinated carbon monoxide by molecular hydrogen is that published by Bercaw et al. (35, 36). They reported that mononuclear carbonyl and hydride complexes of bis(pentamethylcyclopentadienyl)zirconium are capable of promoting stoichiometric H2 reduction of CO to methoxide under mild conditions. Thus, treatment of the dicarbonyl complex (rj5-C5Me5)2Zr(CO)2 with... 

Homogeneous Catalysis of the Water Gas Shift Reaction Using Simple Mononuclear Carbonyls... 

King and coworkers—formate intermediates and a kinetic approach over Cr and other Group VIb carbonyl catalysts. Like Pettit and coworkers,18 34 King et a/.25,3 J 42,43,54,59,138,155 also studied water-gas shift on mononuclear carbonyls of iron and, in a detailed investigation, indicated that Scheme 7 of Pettit et al,34 was... 

For a mononuclear carbonyl, M(CO) , the enthalpy of disruption AHD refers to the process... 

Specific reviews of the electrochemistry of mononuclear carbonyls have not appeared. The primary oxidation of the mononuclear carbonyls leads to the formation of 17-electron radical cations with half-lives in the order of seconds or less in MeCN electrolytes [14, 15]. Decay may take place by disproportionation, CO loss, and/or nucleophilic attack. Electrogeneration in solvents of low nucleophilicity such as trifluoroacetic acid can enhance the stability of the cations and indicates that nucleophilic attack is a major pathway for decay. This is concordant with the stability order [Cr(CO)g]+ > [Fe(CO)5]+ [Ni(CO)4]+, where the lower coordination numbers favor nucleophilic attack and... 

Deprotonation with aluminum alkys, 9, 272 mononuclear carbonyl iridium complexes, 7, 302 for palladium cyclopentadienyl complexes, 8, 390 in Ru and Os half-sandwich preparations, 6, 569 in silver carbene synthesis, 2, 206 Desulfurization... 

The appearance potential of CO+ in the spectra of mononuclear carbonyls is similar to the ionization potential of carbon monoxide. This excludes a process... 

We later prepared the more stable phosphine-substituted mononuclear carbonyl hydrides such as FICo(CO)4 nLn and FHVfn(CO)5 L n= 1 and 2 ... 

Although information on the force constants of metal-metal bonds can be obtained from Raman spectra (115), no study of HNCC has yet been reported, probably because these clusters are highly colored and readily decompose in the laser beam. Comparison between several colorless mononuclear carbonyls and the strong colors of HNCC shows that the metal-... 

In mononuclear carbonyls, CO is invariably attached to the metal through carbon giving a linear M C 0 arrangement. Polynuclear carbonyls have relatively short distances between metal atoms indicative of metal-metal bonds. CO can then bond on either a terminal or a bridging mode, the former bonded to one metals as in Mn2(CO)10 (1) and the latter attached to more than one metals as in Co2(CO)8 (2). In larger... 

The Mononuclear Carbonyls When exchanged into the zeolite as Rh(NH )jCll-z+, the rhodium ammine complex could decompose in an oxygen stream at temperatures ranging 150-350°C into a rhodium Ill-hydroxy-species with a partial reduction into diamagnetic Rh(I) and paramagnetic (less than 10%) Rhll species. 

Pyrolysis reactions of mononuclear carbonyls and low-nuclearity cluster compounds have been used extensively in the syntheses of HNCC of osmium (54, 72,80,95,108), ruthenium (18,20,29), and, more recently, rhenium (2-4). The reactions have been carried out either in inert solvents or, to facilitate the ejection of CO or other volatile ligands, in the solid state under vacuum. Condensation processes under pyrolytic conditions are rarely specific and, as such, lead to the formation of a wide range of products. In order to obtain optimum yields of a particular HNCC, the reaction conditions must be carefully screened. Solution reactions offer advantages such as the ability to monitor the progress of the reaction using IR spectroscopy. As they often give... 

---