Metal carbonyls, mononuclear polynuclear

The 0 state, d8. The chemistry in this state is primarily one of the metal carbonyls mononuclear and polynuclear carbonyls are known for both... 

Many of the methods used for the preparation of mononuclear hydrides may be applied to the polynuclear systems. Base attack on metal carbonyls, which furnished one of the first methods for the production of carbonyl hydride species, is applicable to a wide range of carbonyls. Borohydride reduction leads to a variety of products, depending upon the reaction conditions, Os3(CO)12 reacting with NaBH in di-oxane under reflux to give, after 4 hours, a mixture of the anions [H30s4(C0)12] and [H2Os4(CO)i2]2 (79). The related reaction in tetra-hydrofuran for 1 hour yields the anion [HOs3(CO)n]- as the main product with minor amounts of the two tetranuclear anions. 

Many of the mononuclear metal carbonyls have been the object of detailed and precise calorimetric measurement by Wilkinson10 and Skinner6 and their respective groups. Measurements on the more complex, polynuclear carbonyls have been made almost exclusively by microcalorimetry4 9, with the notable exception of Good s work12 on Mn2(CO)10. The results of these measurements are collected in Table 1. 

Regardless of the details of the exchange mechanism, it is clear that careful control experiments are necessary when working with deuterated metal carbonyls, be they mononuclear or polynuclear. Also, D2O exchange catalyzed by a chromatographic support may be a convenient method for synthesizing certain metal deuterio complexes. 

In both the examples given above, there is concomitant loss of one or more neutral ligands. Elimination of CO is the rule in reactions of mononuclear metal carbonyls (e.g., entry 12) and cyclopentadienyl metal carbonyls (e.g., entry 4), but not those of polynuclear carbonyls (e.g., entry 16) or carbonyl halides (e.g., entry 33). Elimination of tertiary phosphines often occurs, especially when more than two molecules are present in the initial complex however, this is not always the case (see entry 24). Clearly, steric requirements and the dictates of the 18-electron rule determine the composition of the product, and normally act in concert when they conflict, as in the case of R3SiRuH3(PR3) (n = 2 or 3 entry 22), variable stoichiometry may result. Chelating diphosphines, with somewhat reduced steric requirements, are usually retained (e.g., entry 19), while complexed olefins are invariably lost the bulky ligand P(cyclohexyl)3 is associated with unusual products (entries 47 and 48). Particular mention may be made of the 17-electron species Cl3SiVH(Cp)2 and (Cl3Si)2V(Cp)2 shown... 

There are, however, several aspects of contemporary transition metal chemistry whose existence could not have been extrapolated from the Wernerian principles. Among these one could mention considerable areas of metal carbonyl type chemistry, much of the current field of organometallic chemistry and, most unambiguously, the chemistry of compounds containing metal-metal bonds. Although Werner dealt extensively with polynuclear complexes, these were conceived simply as two or more mononuclear complexes united only by the ligands they shared. 

In marked contrast to the large number of polynuclear metal carbonyl complexes known there are as yet relatively few reports of analogous polynuclear (i.e., containing more than two metal atoms) transition metal-PF3 complexes. Triftuorophosphine can displace up to half the coordinated CO ligands in [Ru3(CO)12] (method A) before the metal cluster is broken and mononuclear complexes are formed. 

Metal elusters are synthesized by this method. Either mononuclear metal carbonyl anions or polynuclear metal carbonyl anions with metal halides may be used ... 

The water-gas-shift reaction catalysed homogeneously in the presence of polynuclear metal carbonyls is of current interest. In some ruthenium systems, the principal species present in basic solutions under reaction conditions of one atmosphere pressure and at 100°C are [HRu3(CO)n] and [H3Ru4(CO)i2] . This has reasonably been taken as evidence to implicate ruthenium clusters as the probable catalysts, although it should be noted that mononuclear systems effectively promote the water-gas-shift reaction. An important finding is that mixed ruthenium-iron carbonyl clusters, e.g.. 

Polynuclear complexes are produced by irradiation of substituted metal carbonyls in analogy to the reaction observed with simple metal carbonyls (see section El). In most cases either CO or another ligand attached to the mononuclear complex serve as bridging functions in the polynuclear products. 

The treatment of many of the metal carbonyls with bases affords complex polynuclear carbonylates (see (140) for a recent review). As is the case for the mononuclear carbonylate anions, hydrolysis of the anions, often with dilute acids, affords diamagnetic metal carbonyl hydride complexes (see Table VI). The majority of these complexes which are described below have not been studied by infrared and proton magnetic resonance spectroscopy their formulations are based primarily on analysis and mode of decomposition, and thus remain tentative. 

Uncharged mononuclear and polynuclear metal carbonyls are known for the group 5 to group 10 metals. The EAN rule see Effective Atomic Number Rule) is normally obeyed, so elements with an even atomic number form mononuclear compounds, for example, Cr(CO)e, Fe(CO)5, and Ni(CO)4, while elements with an odd atomic number form dinuclear compounds, for example, Mn2(CO)io and Co2(CO)g, containing metal-metal bonds. 

All carbonyls are highly toxic by all routes of exposure the volatile liquid compounds present additional inhalation risk. Toxicity may be attributed to their decomposition products, the metals and carbon monoxide, essentially the toxic metabolites of all metal carbonyls. Therefore, the unstable carbonyls, especially the mononuclear complexes, which are readily susceptible to breakdown, should be treated as dangerous poisons. The polynuclear carbonyls of heavy metals may bioactivate and exhibit severe delayed effects. Oral intake of such substances can be fatal. The health hazards associated with individual compounds are discussed in the following sections. Toxicity data for most compounds of this class are not available. 

Soon after the synthesis of mononuclear transition metal carbonyls, polynuclear metal carbonyls featuring bridging carbonyls, such as Fc2(CO)9 and Co2(CO)g, were obtained and cyclopentadienyl anirni containing complexes like [(Cp)Fe (CO)2l2 followed. The first example, Fe2(CO)9, features three identical bridging carbonyls and six terminal carbonyls in a high-symmetiy (D3 ) strucmre that facilitated theoretical treatment and was extensively studied [16—22], though its X-ray stmcture was obtained quite late [4, 23]. These studies examined two fundamental questions (1) what is the electronic nature of Fe-C(0)-Fe three-center bridge and (2) what role does the metal—metal interaction play in the complex, as the short distance between two irons and the 18-e mle imply a metal-metal bond ... 

In a manner similar to that of mononuclear complexes, polynuclear metal carbonyls undergo a fragmentation in which a stepwise loss of carbonyl groups occurs. Generally, the presence of cluster ions of the type is observed, for example, is formed from [M3(CO)i2] where M = Fe, Ru, Os, or Os is formed from Osg(CO)23. Spectra of dinuclear carbonyls, such as [Mn2(CO)io], [Re2(CO)io], [MnRe(CO)io],... 

Main group elements form a wide variety of neutral, cationic, and anionic compounds having chain, ring, and cage structures (e.g. Zintl anions). [366] All of these may be useful starting materials to be combined with mono- and polynuclear transition metal moieties to generate molecules with novel and exciting structural features. This alternative type of synthetic approach has been used for the synthesis of all the other derivatives included in Table 3-11. For example, as shown in Scheme 3-12, the reaction of either Se4 or Te4 with such mononuclear metal carbonyls as [Fe(CO)s] and [W(CO)6] can yield cluster compounds... 

The review of the literature of 1991 follows the sane format as last year, with some general review articles given in section 2. This chapter is further divided into sections for the metals of each periodic group with subdivisions, where necessary, to help the reader locate complexes of basic structural types. Substitution reactions of mononuclear metal carbonyls with group V and/or group VI donor ligands are fully discussed, but metal-metal bonded polynuclear metal carbonyls are only covered when reactions lead to products in which these bonds are cleaved. Carbonyl complexes which contain metal-metal bonds are fully reviewed in Chapter 9. 

A lone pair of electrons are available on both carbon and oxygen atoms of a carbon monoxide ligand. However, as the carbon atoms donate electrons to the metal, these complexes are named carbonyls. A variety of such complexes, such as mononuclear, polynuclear, homoleptic and mixed ligands, are known. These compounds are widely studied due to their abUity to release carbon monoxide [3], their industrial importance, their catalytic properties [4] and their structural interest [5]. Carbon monoxide is one of the most important TC-acceptor ligands. Because of its TC-acidity, carbon monoxide can stabilize the zero formal oxidation state of metals in carbonyl complexes. 

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