Anionic atomic clusters

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. 

In the past decades, homoatomic polyanions in solid state compounds of group 14 elements have been a powerful starting material toward different kinds of chemical transformations. Recent developments, especially for nine-atom clusters, have been discussed elsewhere [5,125,126] and are not within the scope of this review. Here, we concentrate on neat solvates , which means that in addition to cation and anion... 

The tetrahedral Sn2Bi2 ion has been stmcturally characterized by Corbett [72] and studied in solution by Rudolph [23]. The Sn NMR resonance at — 1,574 ppm shows coupling to one other Sn atom with a large, one-bond J( Sn- Sn) = 1,638 Hz. Recent gas phase and theoretical studies on Bi-substituted nine-atom cluster anions suggest that the isoelectronic Sng" analog, SngBis , should be static in solution [31]. 

Further oxidation of the nine-atom clusters to formal [Ge9] leads to linear polymers oi[-Ge9-] with two covalent intercluster bonds (Fig. 2i). Trimers [Ge9=Ge9=Ge9] (Fig. 6a) and tetramers [Ge9=Ge9=Ge9=Ge9] (Fig. 6b) occur via nonclassical bond formation between two neighboring atoms of the triangular prism basis planes of the c/oio-shaped clusters, which results in Ge-Ge-Ge bond angles of 90° and in considerably longer Ge-Ge contacts between the cluster units. Quantum-chemical calculations have shown that the exo-bonds participate in a delocalized electronic system that comprises the whole anion [204]. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

Much of the current literature on metal atom cluster species employs bonding concepts that are derived from MO treatment of the polyhedral borane anions, We thus begin by discussing these species, of which the most important examples are shown in Figure 8.15. We shall deal with the BaHg" ion in detail to illustrate the general approach to these systems. 

Some striking demonstrations of metal-metal bond lability are provided by cluster rearrangements due to protonation. This is the case for some anionic osmium clusters (cf. Section VI). It involves ligand activation for some tetrairon clusters (51-53). Thus, the clusters 9 and 11 open up upon protonation, and compensation for the lost iron - iron bonds in the products 10 and 12 comes from the bonding between one iron atom and a carbonyl oxygen. The relation of these unusual nucleophile-electrophile interactions to cluster-induced CO transformations is obvious. 

Anionic carbonyl clusters with interstitial main-group atoms... 

We will investigate the stability of the anionic lithium clusters in Section 5. A relevant quantity is the dependence of the binding energy per atom on the number of atoms, as well as the electron affinity of neutral Lin clusters. From the latter, we evaluate whether these neutral clusters are able to receive an extra electron and to form an anionic system. 

Binding energy per atom Eb /n (kcal/mol) of the small anionic lithium clusters. 

The anionic tetrahedral clusters [Fe3(CO)9(/u,3-E)]2 (E = S (52), Se (53), Te (48)) can be obtained from the thermolytic reaction of K2E with [Fe(CO)5] in DMF. Although a lone pair of electrons is present on E, investigations of the reactivity of electrophiles and oxidants toward the lone pair of electrons on the apical Te atom in 48 suggest strongly that the apical Te atom is unreactive toward electrophiles and that reaction occurs at metal centers (Scheme 3).50... 

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