Polynuclear Carbonyl Derivatives

3 Polynuclear Carbonyl Derivatives.-Two major problems in cluster chemistry were addressed in 1985. As cluster carbonyl species have become larger, the description of the spatial distribution of metals and ligands have become a nontrivial problem. A relatively simple system has been proposed which introduces new notation to define these parameters unequivocally. The largest molecular 

3 Polynuclear Carbonyl Derivatives.-Variable temperature MAS n.m.r. methods have been used to probe the exchange processes occurring in solid homonuclear carbonyls containing 3 and 4 metal atoms.A quantitative investigation of the chemistry resulting from both continuous- and flash-photolysis of Ru3(C0)i2 has been carried out, and a new polymeric ruthenium carbonyl, Ru(C0)it n prepared from this precursor. The steric and electronic effects in associative substitution reactions of Iri (C0)i2 have also been examined. The combination of two OS3 units to yield an improved route to Ose raft-like clusters has been achieved, and a comprehensive analysis published on the different Mg cores of stacked Mg triangles in the [Mg C0)i8]  

Heteronuclear cluster carbonyl containing non-transition metals aroused considerable interest in 1986. Following the preliminary account of the structure of [Bii,Fei,(C0)i3]2 , which was described in the previous report in this series, this electron-rich Zintl-metal carbonylate has now been the subject of a detailed theoretical discussion. i The attempted alkylation of a bridging 

CO in a cluster that also contains a Bi atom has been reported, and efforts made to synthesise further Zintl-metal carbonylates by reaction of Bi2Fe3(C0)g with other metal carbonylates. In the course of these studies, the structures of [Fe3(C0)9 M-H)3(M3-Bi)], 2 EM[Bi2Fe2Co(CO)io] and (EUN)2[Bi2Fe (C0)i3] were elucidated by JT-ray methods. In related studies the structure of the electron-deficient species (Ett,N)6[Tl6Feio(CO)36] has been described and the 

The latter contains very long Ir-Ir bonds (3.086 A), suggesting very weak, if any, metal-metal interactions. Further studies on metal carbonyl derivatives containing or Te ° have been described. These include details of 

Detailed vibrational studies have been carried out on several carbonyl cluster compounds. Both single crystal and Raman solution data have been obtained for M3(C0)i2 (M = Ru or Os), and small changes in frequency of the two-mode v(C0) A vibrations of these two carbonyls in mixed crystals have been used as indicators of the molecular geometry changes by which each species adapts to sites in the mixed crystal. Intermediates formed on the interaction of several Group VIII cluster carbonyls with oxide surfaces have also been characterised by spectroscopic means. 

In addition to papers of a theoretical nature which have been referenced in Section 2, a great many structural studies have been carried out on new and known clusters. Many of these complexes have been prepared using the known tendency of 0- or S-donor ligands to labilize coordinated CO groups, and hence lead to the formation of metal species of high nuclearity. 

The structure of the first example of a peripheral dicarbido metal carbonyl cluster has been announced. [Coe(u6 C2)(y-CO)e(CO)e-(ui,-S)] is formed in low yields from the reaction of Co2(C0)8 with CS2. The Co6 unit assumes a boat configuration with the C2 unit 

A delocalised rather than a localised C-C interaction is considered probable.  

A new family of nitrido carbonyl cluster anions [Rh6MN(C0)is] 

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The infrared spectra of certain polynuclear carbonyl derivatives in solution contain more CO-stretching frequencies than would be expected on the basis of symmetry considerations. Such an effect has been attributed to the presence of more than one isomer in solution. Thus, eleven terminal and two bridging CO-stretching frequencies were observed in the infrared spectrum of the compound Co2(CO)g in solution (47, 246). At low resolution, not all these bands were observed, and consequently initial structure determinations were incorrect (54, 74, 99,148). Bor used... 

At first glance, the structures and stoichiometries of polynuclear carbonyls and their derivatives are of baffling complexity. As we shall see in the next section, some very simple electron book-keeping devices help to bring some order from the apparent chaos. 

C. Mass Spectra of Polynuclear Metal Carbonyl Derivatives. 95... 

Unlike cobalt and rhodium, the chemistry of polynuclear iridium carbonyl derivatives has not been studied in detail (15a). Reduction of Ir4(CO)i2 under carbon monoxide with K2C03 in methanol gives the yellow tetranuclear hydride derivative [Ir4(CO)nH], whereas under nitrogen the brown dianion [Ir8(CO)2o]2- has been isolated as a tetraalkylam-monium salt (97). It has been suggested that the structure of the dianion could result from the linking of two iridium tetrahedra, although its formulation so far is based only on elemental analyses. Clearly such an interesting compound deserves further chemical and structural characterization. 

The principle of reacting a silicon hydride HSiR3 with a polynuclear carbonyl (or carbonyl cyclopentadienyl derivative) was successfully applied to other transition metals such as Mo 37 Mn40,137 81,207 Re135> 137> 207),... 

Metal-Metal bond enthalpy contributions, M/kJ mol in polynuclear metal carbonyls derived from a) two-centre electron pair bond description, b) bond enthalpy/bond length relation. 

Structural chemistry of it complexes 69 of transition metals with carbonyl (158) ligands. I. Mononuclear and polynuclear carbonyls and their derivatives without metal-metal bonds... 

Today, however, polynuclear carbonyls are of primary interest. Their structure is frequently so complicated that it cannot be derived from the vibrational spectra by applying selection rules. Only certain structural elements, such as terminal and bridging CO groups, can be identified. Reliable information about the structure of these species can only be obtained by X-ray diffraction methods. 

S.8.5.2.2. by Reaction with Polynuclear Transition- or Inner Transition-Metal Carbonyl Derivatives. 

The carbonyls are in general volatile compounds with an extensive chemistry which presents many problems as regards valence and stereochemistry. Some are reactive and form a variety of derivatives, as shown in Chart 22.1 for the iron compounds, while others are relatively inert, as for example, Cr(CO)6 etc. and Re2(CO)iQ. This rhenium compound, although converted to the carbonyl halides by gaseous halogens, is stable to alkalis and to concentrated mineral acids. A few carbonyls may be prepared by the direct action of CO on the metal, either at atmospheric pressure (Ni(C0)4) or under pressure at elevated temperatures (Fe(CO)s, Co4(CO)i2)- Others are prepared from halides or, in the case of Os and Re, from the highest oxide. The polynuclear carbonyls are prepared photo-synthetically, by heating the simple carbonyls, or by other indirect methods. 

The heavy metal derivatives of H2Fe(CO)4 and HCo(CO)4, the so-called mixed metal carbonyls , show very considerable differences in properties. HgFe(C0)4 is a stable yellow solid, insoluble in both polar and non-polar solvents these properties suggest a polymeric structure for example, of type (a), whereas the derivatives of HCo(CO)4, M[Co(CO)4]2, where M = Zn, Cd, Hg, Sn, or Pb, resemble the polynuclear carbonyls, being soluble in non-polar solvents, insoluble in water, and subliming without decomposition they consist of simple linear molecules in the crystalline state (see later). 

Polynuclear transition-metal carbonyl derivatives are among the most accessible cluster compounds. This class of compounds also has the advantage that their re-... 

These and related methods of structure prediction depend upon the allocation of specific electron counts to different framework geometries and these applications have been very successful in assigning structures to cluster carbonyls and their derivatives. However, for the higher polynuclear carbonyls as the molecularity of the compounds increases the predictive power of the theories becomes less decisive in differentiating between alternative structures. In essence the Wade-Mingos approach assumes that the frontier orbitals of the complex primarily involve metal orbitals so that any variation in the electron occupation will be reffected in a structural change in the metal framework. The Wade theory also requires that the structure of the complexes are based on triangulated polyhedra as found for the boron hydrides. 

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