Transition Metal Clusters, Including Dimers

Cleavage of C—H bonds is also well documented for mononuclear complexes (66-68, 147), and seems likely for dinuclear systems [(84, 386) also Section II,A, Eq. (7)] but apart from some cyclopropane systems (387), cleavage of C—C bonds has not been demonstrated at mononuclear centers. Equation (65) shows such cleavage at a cluster center (388)  

In terms of hydrogenation, solutions of H2Os3(CO)i0 reduce ethylene stoichiometrically, and the resulting unsaturated intermediate Os3(CO)10 can oxidatively add H2, or further ethylene to give a hydridoalkenyl 

The tetrahedral nickel isonitrile cluster Ni4(CNR)7 (R = cyclohexyl or f-Bu) contains three bridging isonitrile ligands and a terminal one at each nickel (52) the bridging ligands are mobile on the cluster surface and a ligand dissociation step occurs (33, 390, 391). 

The cluster catalyzes hydrogenation (20°C and 3 atm) of dialkyl- and diarylacetylenes to the c/s-olefins via unsaturate routes, likely involving Ni4(CNR)6(RC=CR) and Ni4(CNR)4(RC==CR)3 (391, 392). The acetylenes in the latter complex bridge three nickel centers, and increase of the acetylenic carbon-carbon bond distance is considered to enhance reduction by hydrogen (392, 393). 

The tert-butyl cluster Ni4(CNR)7 also catalyzed slowly the selective H2 reduction of the isocyanide to ferf-butylmethylamine (394). A problem arose in that the excess isocyanide resulted in formation of Ni(CNR)4, which is relatively inactive, but this was circumvented by using for the isocyanide source a buffered solution containing Ni4(CNR)7 and Ni(CNR)4 in a 1 10 ratio  

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Cluster compounds have been included iu previous sections of this chapter and earlier chapters. Transition-metal cluster chemistry has developed rapidly since the 1980s. Beginning with simple dimeric molecules, such as Co2(CO)g and Fe2(CO)9, chemists have developed syntheses of far more complex clusters, some with interesting and unusual structures and chemical properties. Large clusters have been studied with the objective of developing catalysts that may improve on the properties of heterogeneous catalysts the surface of a cluster may mimic the behavior of the surface of a solid catalyst. 

There have been numerous photodetachment studies of small cluster anions, and we now give some examples. Noble metal clusters (Cu, Ag,7, Au , n = 1-10) have been studied by Ho et al. [23], who resolved vibrations in all three dimers. Studies of alkali metal cluster anions have included those of Na ( = 2-5), K (n = 2-19), RbJ 3, and CS2-3 [24,25]. Carbon cluster anions C,T have photoelectron spectra that are consistent with linear chains for n = 2-9 and monocyclic rings for n = 10-29 [26]. Photoelectron spectra of Sb and Bi to n = 4 [27] show rich vibrational structure for the dimers, and the spectra of the larger clusters could be interpreted in terms of ab initio calculations. The threshold photodetachment (zero electron kinetic energy, ZEKE) spectrum of Si4 [28] shows a progression of well-resolved transitions between the ground state of the rhombic anion (Dzh, and vibrational levels of the first excited... 

A couple of recent papers on different transition metal dimers are discussed here. Buchachenko et al. [36] studied a different dimer, Mnj, and showed that this is also a typical challenging case for quantum chemical methods. They performed calculations by employing a set of methods including coupled cluster, multireference configuration interaction (Cl) methods, and CASPT2. They showed that different methods predict different electronic structures for this system and tried to rationalize the differences among different methods. 

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