Some Mixed Clusters

The foregoing examples show the relevance to metal-carbonyl cluster chemistry of the borane-oarborane structural and bonding pattern. Its relevance to other areas of chemistry may be explored readily using a systematic skeletal electron-counting procedure (161, 201). 

The Number of Skeletal Bonding Electrons (v + x — 2) That Main Group Cluster Units Can Contribute 

For mixed main group-transition element clusters containing but one transition element, modifying the above electron-counting procedure to include the 6 nonbonding electron pairs on the transition metal has been advocated (128) as a way of bringing the electron count into line 

Large numbers of metallo-boranes and -carboranes (31h, 68a, 94a, 95, 96a, 96b, 172a, 183, 195, 205b, also Williams, this volume) are now 

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Because of the large number of carbonyl groups required to satisfy the noble gas rule and the relatively small metallic radius, serious steric limitations are present for manganese tetranuclear closed clusters12). The only known tetranuclear compounds are some mixed clusters, MnOs3(CO)16H and MnOs3(CO)13H3 (see later), which have been prepared from Os3(CO)12 and [Mn(CO)s] 161). 

FlO. 21. Skeletons of some mixed clusters with 8 skeletal bond pairs. 

Fig. 22. Some mixed clusters with 7 skeletal bond pairs.

Table 15 Some Mixed Clusters Containing One or More Gold Atoms...

Simple cations are unknown within Group 16 (besides Po), but several highly colored polyatomic cations (cationic clusters), like S " ", Sg, Se, SCg, Te, and Teg" ", have been isolated in non-aqueous media [15]. Some mixed chalcogen cationic clusters have also been reported. These are all unstable in water. 

Mixed clusters NH3/H20 (139-141), NH3/MeOH (61), and NH3/Me2CO (142) have been reacted with bare metal ions and in general the transition metal ions preferred coordination to ammonia whereas the non-transition metal ions such as Mg+ and Al+ were nonselective, showing some similarity to condensed-phase systems. 

There are several examples of mixed clusters which are homologous with the dode-cacarbonyls. Some of these compounds, such as the anion [FeCo3(CO)i2] and the neutral cyclopentadienyl carbonyl Co3Ni(CO)9Cp, are believed to be iso-structural with Co4(CO)j2 and it is therefore convenient to discuss these compounds now. 

A rather obvious means of Incorporating the electron-poorer elements from groups earlier In the periodic table Into clusters Is then to combine them with electron-rlcher elements. Some results of synthetic and structural Investigations of this sort Involving Ions with 2.2 - 3e per cluster element are listed In Table II together with data for three electron-rlcher mixed cluster anions. 

Since the completion of this review (mid-1982), the chemistry of carbidocarbonyl clusters has continued to expand rapidly. The task of the reviewer is made even more difficult as fascinating results continue to appear. In resisting the temptation to make a comprehensive update of the field, it would be remiss of me not to direct the reader s attention to the continued investigations of Lewis, Johnson, and co-workers in the chemistry of ruthenium and osmium carbidocarbonyls (89), the report by Longoni and coworkers (90) of the syntheses of the first nickel carbide clusters and some mixed nickel-cobalt carbides, the syntheses by Shapley of a new ruthenium dicarbide cluster [Ru,oC2(CO)24]- (91) and of Os6C(CO)l7 (92), and the work of Shriver which implies the existence of a very reactive tri-iron carbide cluster (93). 

This restriction rules out all discrete models exclusively based on semiempirical force fields, leaving among the discrete models the MC/QM and the MD/QM procedures, in which the second part of the acronyms indicates that the solute is described at the quantum mechanical (QM) level, as well as the full ab initio MD description, and some mixed procedures that derive the position of some solvent molecules from semiclassical simulations, replace the semiclassical description with the QM one, and repeat the calculation on these small supermolecular clusters. The final stage is to perform an average on the results obtained with these clusters. These methods can be used also to describe electronic excitation processes, but at present, their use is limited to simple cases, such as vertical excitations of organic molecules of small or moderate size. This limitation is due to the cost of computations, and there is a progressive trend toward calculations for larger systems. 

Skeletal structure of some mixed Au/Ag clusters (filled circle, Au open circle, Ag) (a) [Au13Ag12] in [(Ph3P)ioAui3Agi2Br8]SbF6,... 

Melnik and Parish (10) correlated 197Au Mossbauer and X-ray structural data for gold clusters in 1986, and Salter (11) has recently published a review of the stereochemical nonrigidity exhibited in solution by the metal skeletons of some mixed-metal clusters containing copper, silver, and gold. 

Another technique that has recently proved to be very useful for facilitating the characterization of heteronuclear Group IB metal cluster compounds is fast atom bombardment (FAB) mass spectrometry. Although the mass spectra of some mixed-metal clusters containing ML units have... 

We may note, finally, that nickel and platinum form various mixed clusters, some very large, as in the case of the [Ni3gPt( (CO)4gHmedium size cluster, [Nic,Pt3(CO)2iH]3 , has been structurally characterized. The metal framework is as shown in (16-XXV). 

One of the most important links between alkylidyne and alkyne compounds is that one of the first synthetic routes for cobalt al-kylidynes involved alkynes as reagents (264-268). In later studies, several other synthetic routes to cobalt (269-280), rhodium (281, 282), iron (283-285), molybdenum (286, 287), ruthenium (288-292), osmium (293, 294), nickel (295, 296), and some mixed-metal (165, 297-302) clusters have been developed. Reagents employed include carbynes (166, 277, 280), alkali metals (269), carbon disulfide (275), dithioesters (276, 282), RCC13, and acids (281, 282). 

Adams et al. [78,79] have reported a series of synthesis of mixed-metal cluster compounds. One example, Pt2Ru4(CO)is, is depicted in Figure 1(b). This mixed cluster compound was investigated to study the effect of Pt-Ru nanoparticles developed after the precursor annealing on carbon [80]. In line with the spectroscopic and microscopic measurements, the authors demonstrated that mixed Pt-Ru nanoparticles, with an extremely narrow size distribution (particle size 1.4nm), reflect an interaction that depends on the nature of the carbon support. Furthermore, as revealed by EXAFS, the Pt-Pt, Pt-Ru, and Ru-Ru coordination distances in the precursor (2.66, 2.64, and 2.84 A) [79] changed to 2.73, 2.70, and 2.66 A, respectively, on the mixed-metal nanoparticles supported on carbon black, with an enhanced disorder [80]. Furthermore, some metal segregation could be... 

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