Bare metal cluster cations

There are several kinds of cluster ions, both cations and anions, observed in mass spectrometers. There are bare metal cluster cations and anions, binary clusters cations and anions M Em (where E is an element such as O or S), and other clusters involving ligands and metals. In this section, the bare metal cluster cations Mj and anions M will be discussed separately followed by the binary cluster cations M E+, then binary cluster anions M E , and finally other cluster systems having more than one metal atom. 

The study of bare metal clusters is central to the understanding of the links between solid state chemistry and that of discrete molecular species. Alkali metal clusters have been studied in molecular beams [12, 13], and the theoretical models proposed have attempted to interpret the abundances observed in the mass spectra of these clusters. These spectra show large abundances for specific numbers of metal atoms (N), the so-called magic numbers . Neutral alkali metal mass spectra show peaks at N = 2, 8, 20,40, 58, whereas cationic species show large abundances at N = 19, 21, 35, 41. The theoretical study of alkali metal clusters is simplified by the presence of only 1 valence electron per atom. 

The sections are divided by the coordination number of the reacting ion defined as the number of donor atoms that interact with the metal. The nomenclature used for the ligands is L for neutral molecules that act as ligands and X for anions that act as ligands. Most of the examples in this section will involve cations [ML ]+ or [MX ]+, but there will be a short section on bare metal anions, M . The anions of more complexity than M will be discussed in Section IV on clusters. Many reactions produce an initial product that continues to react resulting in further coordi-native changes and possibly redox changes. Tables I and II will indicate the initial reaction product and other major reaction products. 

Modem work on these and related bare post-transition element clusters began in the 1960s after Corbett and coworkers found ways to obtain crystalline derivatives of these post-transition element clusters by the use of suitable counterions. Thus, crystalline derivatives of the cluster anions had cryptate or polyamine complexed alkali metals as countercations [8]. Similarly, crystalline derivatives of the cluster cations had counteractions, such as AlCLj, derived from metal halide strong Lewis acids [9]. With crystalhne derivatives of these clusters available, their structures could be determined definitively using X-ray diffraction methods. 

Fig. 6 compares the nuclearity effect on the redox potentials [19,31,63] of hydrated Ag+ clusters E°(Ag /Ag )aq together with the effect on ionization potentials IPg (Ag ) of bare silver clusters in the gas phase [67,68]. The asymptotic value of the redox potential is reached at the nuclearity around n = 500 (diameter == 2 nm), which thus represents, for the system, the transition between the mesoscopic and the macroscopic phase of the bulk metal. The density of values available so far is not sufficient to prove the existence of odd-even oscillations as for IPg. However, it is obvious from this figure that the variation of E° and IPg do exhibit opposite trends vs. n, for the solution (Table 5) and the gas phase, respectively. The difference between ionization potentials of bare and solvated clusters decreases with increasing n as which corresponds fairly well to the solvation free energy of the cation deduced from the Born solvation model [45] (for the single atom, the difference of 5 eV represents the solvation energy of the silver cation) [31]. 

CID methods have proven to be very useful in measuring the stabilities of clusters of bare transition metal atoms, providing many more thermochemical values than photodissociation methods. In our laboratory, we have used CID to study the cationic clusters of ten different transition metal elements, including TiJ (x=2-22),VJ (x=2-20), CrJ (x=2-21),Mn, FeJ (x=2-19),CoJ (x=2-18),NiJ (x=2—18), and CuJ the second row transition metal clusters of NbJ (x=2-ll) and the third row transition metal clusters of Taj (x=2-4). These results have been summarized and trends analyzed previously [176,177]. CID methods have also been used by Ervin et al. to measure the stabilities of anionic clusters of the coinage metals Cu (x=2-8) [178], Ag (x=2—11) [179], and Au (x=2-7) [180] and group 10 metals Pd [181] and Ptx (x=3-6) [182]. A multiple collision-in-... 

Much of our effort involves studies of the chemical behavior of dusters not only as a function of size, but also as a function of metal type, charge state (neutral, cationic or anionic), and reagent molecule. There are two different operating conditions for which we probe the chemisorption of molecules onto clusters as a function of duster size. The first is such that the rate of reaction is kinetically controlled. Here we obtain information about the rate at which the first reagent molecule chemisorbs onto the otherwise bare cluster. In the second case, chemisorption studies are carried out under near steady-state conditions. In this instance we attempt to determine how many molecules a particular size cluster can bind, i.e. the degree of saturation. 

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