Polynuclear carbonyls structures

Most of the metals which form polynuclear carbonyls adopt the 12 co-ordinate face-centred cubic structure ( = 12) in the solid state so that the relation between Aland T can be revised to... 

In this chapter we have examined examples of polynuclear metal carbonyl complexes as well as simple metal carbonyl hydrides. Consider now the polynuclear carbonyl hydride complex, H,Os,(CO)i2- Rationalize the formulation of this species. From your application of the 18-electron rule, what can you say about the structure of this molecule1 How is it similar to or different from the complex Os COln shown in Figure 5.9 (See Churchill M. R. Wasserman, H. J. Iticrg, Chen. 1980, 19, 2391-2395.)... 

Binuclear and polynuclear carbonyl complexes were among the first compounds containing M-M bonds to be prepared early in the present century, but it was not until the crystal structure determination of Mn2(CO)i0, i.e. (CO)5Mn—Mn(CO)5, in 1957 that such bonds were authenticated in carbonyls. The study of polynuclear carbonyl complexes has been a major growth area of chemistry since then. Clusters containing more than 50 M atoms have been characterised. Some examples are illustrated in Fig. 8.8. 

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. 

The 18-electron rule can be applied - with some adjustments - to the structures and stoichiometries of polynuclear carbonyl complexes. To illustrate the principles, let us go back to the simple case of Mn2(CO)10. We found that the Mn atom was one electron short of 18 and that formation of the Mn-Mn bond - giving each Mn atom a share in an extra electron - would make up the deficit. In the general case of an M cluster, we can say that N, the number of two-centre, electron-pair M-M bonds, is given by ... 

Transition metal ions, within the zeolite framework, may undergo a reductive carbonylation to give mononuclear monovalent carbonyl coumpounds M(I)(CO) and ultimatly to give zerovalent polynuclear carbonyl clusters. The rhodium(I)and iridium(i)carbonyIs were identified using spectroscopic and volumetric methods, the zerovalent rhodium and Iridium clusters M (CO)j were also synthetized in the zeolite matrix and their structure investigated using IR, NMR and spin labelling methods. 

Complexes in the 0 oxidation state may be obtained either by reduction of halides such as RuCl2(PPh3)3 with Na or Zn in the presence of CO or other ligands such as RNC, or by reaction of metal carbonyls with phosphines. Reactions of polynuclear carbonyls such as Ru3(CO)12 (Section 18-F-13) with phosphines tend to preserve the cluster structure. 

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. 

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... 

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). 

Unlike mononuclear carbonylates, reaction of carbocation reagents with anionic polynuclear carbonylates often results in coordination at the bridging carbonyl oxygen rather than at the metal center. This type of interaction was first observed for the reaction of [HFe3(CO)II] with CH3S03F (88) [Eq. (12)], and a higher yield route is indicated in Eqs. (13) and (14). The X-ray crystal structure of this molecule is shown in Fig. 13. 

There are several examples of polynuclear carbonyls containing single carbon atoms intimately buried in the metal atom cluster. There is as yet no definite idea as to the origin of these carbon atoms. The compounds are usually obtained by refluxing simpler polynuclear carbonyls in hydrocarbons. The two best characterized examples are Fe5(CO)15C and Ru6(CO)17C, whose structures are shown in Fig. 22-6. 

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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