Homonuclear clusters

Apart from a short study on C03Q6 where Q = ethanolamine or 2-aminobutanol [35], only Co4(CO)i2 has been widely studied. The NMR of Co was used to both elucidate the structure and symmetry of the molecule in solution and to investigate carbonyl exchange. 

Early [36,37] and later [29,38] experiments in solution give two Co resonances at 5( Co)= —715 assigned to the apical cobalt atom and —2065 for the basal ones (reference K3[Co(CN)6]). The intensity ratio is 1/3 and the corresponding linewidths are 4 and 12 kHz [29]. All these results confirm the C3V symmetry of Co4(CO)i2 in solution. These values are also found in the solid state [27,28]. 

Cobalt-59 NMR appeared to be of no great help in the study of carbonyl exchange which was elucidated using C NMR in solution at variable temperature and in the solid state [28,39,40] (see also Chapter 6). 

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In addition to the heteronuclear clusters considered in the preceding paragraphs. As, Sb and Bi also form homonuclear clusters. We have already seen that alkaline earth phosphides M3P14 contain the [Pv] cluster isoelectronic and isostructural with P4S3, and the analogous clusters [Asy] " and [Sbv] have also been synthesized. Thus, when As was heated with metallic Ba at 800° C, black lustrous prisms of BasAs were obtained, isotypic with Ba3Pi4 these contained the [Asv] anion with dimensions as shown in Fig. 13.25(a).Again,... 

The nuclearity of structurally characterized homonuclear clusters goes from four to 39. The preparation of [Au55Cl6(PPh3)i2] from the reduction of [AuCl(PPh3)] with B2H6 has also been reported.46,3217... 

Dyson, P. J. Mingos, D. M. P. Homonuclear Clusters and Colloids of Gold Synthesis, Reactivity, Structural and Theoretical Considerations. In Gold, Progress in Chemistry, Biochemistry and Technology. Schmidbaur, H., Ed. Wiley Chichester, U.K., 1999 pp 511-556. 

One of the first published cluster compounds of the heavier group 13 elements was the closo-dodecaaluminate K2[Ali2iBui2] 54 (Figure 2.3-10) [79], which possesses an almost undistorted icosahedron of 12 aluminum atoms with short Al-Al distances (268-270 pm). Up until today, it remained the only homonuclear cluster compound of the elements aluminum to indium which, with respect to structure and cluster electron count, is completely analogous to any boronhydride (see Chapters 1.1.2, 1.1.3, 1.1.5.2, and 2.1.5.6) (in this case doso-[Bi2H12]2 ). Compound 54 was formed in small quantities by the reaction of di(isobutyl)aluminum chloride with potassium and was isolated as dark red crystals (Figure 2.3-10). 

Homonuclear clusters of the heavier elements of the third main-group aluminum, gallium, indium and thallium having direct element-element interactions form a fascinating new class of compounds. As discussed in the previous Chapter 2.3, in some cases their structures resemble those known with the lightest element of that group, boron, while in other cases novel, metal-rich compounds were obtained which do not have any analogue in boron chemistry. 

Most of the reported reactions between tetranuclear clusters and alkynes involve mixed-metal cluster species. In these systems hydride and carbon monoxide substitution generally occurs [Eq. (11)] (194-200), although in some cases Me3NO has been used to activate the starting material (201, 202), and in still others cluster breakdown takes place even under mild reaction conditions (203). Rh4(CO)12 (204) and Ir4(CO)12 (205) retain their nuclearity in reactions with alkynes, but in the latter case the metal framework geometry is altered (Fig. 7). The use of [Ir4(CO)11Br] instead of Ir4(CO)12 in reactions with alkenes produces alkene-substituted tetranuclear complexes (189), as shown in Fig. 7. Few other homonuclear clusters have been found to react with alkynes (206-208). In the reaction between the tetranuclear cluster Cp2W2Ir2(CO) 0 and diphenylacetylene two independent processes... 

The study of small, homonuclear clusters of atoms Is Important In understanding nucleatlon because such clusters are Intermediates In the formation of bulk condensed phases. The dynamic process of condensation from a gas must Initially Involve the formation of tiny aggregates of the new phase. This can be Illustrated by the reaction sequence A(g)—A2(g)— A3(g)— . . . — A(1). One of the major weak points In the present day understanding of such nucleatlon phenomena Is the unknown thermodynamic properties of clusters. Certainly, the common practice of treating a 2-200 atom cluster as a tiny piece of the bulk with a large surface Is Inexact. There Is a need for precise thermodynamic data on atomic and molecular clusters to better define nucleatlon kinetics. 

In this paper we will apply our dimensional model (4) to the problem of determining standard entropies for small homonuclear gas clusters. The model can also be used to determine free energy functions. Our approach will be statistical mechanical in nature and less empirical than the entropy correlation by Bauer and Frurlp. In section II, the dimensional model is reviewed and its application to molecules of the type MXj, 2 n 6 is discussed. In section III, the model is tested for homonuclear clusters, A, by comparing literature data with corresponding data on type... 

The applicability of the dimensional model to Ionic and nonlonlc molecules containing as many as seven atoms Is gratifying. This demonstrates the Insensitivity of the entropy (as determined by the configurational Integral) to the exact form of the pair potential. This encouraged us to attempt similar correlations with homonuclear clusters. We will test whether the Xq species follow the prescriptions of the dimensional model for the corresponding MXjj species. For convenience, we will restrict ourselves to the entropy at 1000 K. The correlations at other temperatures and for the free energy functions are similar. 

Larger Homonuclear Clusters. As one might expect, the scarcity of data on homonuclear polymers increases with cluster size. To date we have only been able to locate published entropy data on four pentamers 85, 805, and Te5 from Mills compilation (7) and C5 from the JANAF tables (6). Furthermore, only data on the hexa-mers and heptamers of 8 and 8e exist (7). All of these data have been estimated by various means. These entropy data are plotted In Fig. 5 against the appropriate form of the logarithmic term In Eq. 3. Please note that the dimensionless entropy scale Is shifted for each polymer type to facilitate comparison. Also plotted In Fig.5 are the least squares fits of unit slope from the corresponding dimensional model tests for the MX ] species (Table 1). These are shown as the solid lines In Fig. 5 and represent the results for MX4 on the pentamer plot, MX5 on the hexamer plot, and MXg on the heptamer plot. 

The results of the testing of the dimensional theory for homonuclear clusters, Xn (n 7). Indicates a general agreement with the model. Unfortunately an accurate and meaningful comparison of the data on clusters larger than trlmers Is severely limited by the lack of accurate experimental spectroscopic data. The uncertainties Involved In the standard estimation procedures are... 

The comparison of the Xj, thermodynamic data with the corresponding MXn x data also appears to suggest a viable method for making predictions from the dimensional model. In every case tested except the Xy species, the MX -i line lies close to the points for the Xj, polymers. We believe that the disparity In the Xy case Is probably due to the lack of enough MX5(g) data points (Fig. 1). Note In Table I that for every species tested except the MXg molecules the Oq values at 1000 K lie close to 20. Interestingly, If we artificially Increase the value for the MX5 from 17.9 to 20, the discrepancy between the two Xy points and the MX5 line Is reduced considerably (Fig. 5). Ultimately we plan to numerically evaluate the configuration Integral with various potential functions for homonuclear clusters. 

Ga-Ga bond distances reside within the somewhat narrow range of 2.605 to 2.648 A. As the authors stated, this was the first example of two tetrahedral R3M units linked by a single metal-metal bond in a homonuclear cluster. 

Ruthenium is an element with two NMR active isotopes Ru and Ru, but only the former is of practical use since it has the lowest quadrupole moment. Ruthenium-101 has a better natural abimdance but the linewidths are about 33 times larger than with Ru. The observability of Ru is three times lower than that of Mo. Only two homonuclear clusters have been reported Ru3(CO)j2 in C6D6 5( Ru)=-1208 with Avi/2 = 80Hz (reference K4[Ru(CN)6] [42] and [HRu3(CO)ii] N(PPh3)2 with two resonances, one at 5( Ru)= —633 and the second at 6( Ru)= —1202 in intensity ratio 2/1 [43]. 

All nonmetals add to the alkaline earth metals with two noteworthy exceptions Mg does not react with elemental carbon, and Be fails to react with H2. A review of homonuclear, cluster-type compounds, also including group IIA metal-nonmetal compounds, is available. The reaction of Mg with H2 requires rather drastic conditions unless homogeneous catalysts are present (see also 10.2.3.3.1). 

The chemistry of heteronuclear cluster compounds is significantly less developed than that of homonuclear clusters. Since 1980, rational synthetic methods [1] and effective characterizational techniques have been developed [2-6], Much of the interest provoked by these molecules stems from the fact that they have interesting chemical properties and potentially unique catalytic properties arising from the synergic effects of polar metal-metal bonds [6]. 

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