Bare Metal Clusters

5 Discrete and Condensed Transition Metal Clusters in Solids 

For the alkali metal clusters in the solid state, only a small island of stability exists so far, and this is with the suboxides of the heavy alkali metals Rb and Cs [4] at relatively low temperatures. Similar compounds of the lighter metals and the higher homologues of oxygen do not exist. 

The more metal rich suboxides may be interpreted as intermetallic phases of the duster metals [Rb902] and [C nOj] with excess Rb and Cs, [4] as in [Rb902]Rbj, [Cs 03]Cs,o, and [Csu03]C. The molar volumes of these phases closely correspond to the sum of the molar volumes of Rl 902 or Csi]03 and the atomic volumes of Rb and Cs respectively. To take an example, the molar volumes of Cs 03Cs and Cs,i03Cs,o exceed the molar volume of Csn03 by 69.9 and 696.5 cm mol respectively, and can be compared with the 69.4 cm mol volume of elemental Cs at 223 K. Qearly, the Cs atoms in these suboxides form purely metallic bonds to the [Cs 03] cluster. 

When Rb-Cs alloys of appropriate composition are partially oxidized, then the two metals which are so much alike in their aqueous chemistry exhibit an entirely different behavior. They beeome spatially separated in the suboxides, whereby the Cs enters into [Csn03] clusters and the Rb is distributed in the purely metallic regions between the elusters. Examples are Csu03Rb (n = 7, 2, 1). [4] There is evidence for the existence of K-Cs mixed suboxides that should, according to what has been said about the Rb-Cs suboxides, have the general comporition Cs,03K or Cs,i03(Cs, K) . Unfortunately, they decompose at 215 K under K2O deposition so that only preliminary X-ray and thermal analysis data have been obtained to identify their existence. 

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The extent of a reaction in these measurements is determined by bare metal cluster ion signal depletion. In most cases products are also observed. Some systems show multiple adducts indicating comparable or higher rates for each successive step up to a saturation level. For other systems the fully saturated product is observed almost as soon as the reaction starts. This later behavior is characteri sti c of an early rate-limiting step. Due to this complexity kinetics have only been reported on the formation of the first adduct, i. e. for the initial chemisorption step. 

Geometric structure of the bare metal clusters and the complexes formed by reaction are unknown and present a significant experimental challenge. Chemical studies are starting to imply something about the structure of the products and will be invaluable until more direct chemical physics probes are available. 

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. 

There are several preparative methods for the production of bare metal clusters including the fast flow reactor (PER), the fast flow tube reactor (FTR), the SIDT (24), the GIB (23), and a supersonic cluster beam source (SCBS) (198). Essentially, all of these methods are similar. The first process is to vaporize the metal sample producing atoms, clusters, and ions. Laser vaporization is generally favored although FAB or FIB may be used. The sample is located in a chamber or a tube and so vaporization generally takes place in a confined environment. An inert gas such as helium may be present in the vaporization source or may be pulsed in after the ionization process. 

In general, there are three types of interactions of ligands with bare metal clusters. 

Platinum oxide cluster anions [Pt O]- and [PtM02] have been prepared by the reaction of the bare metal cluster anions [Pt ]- with N20 and 02, respectively (255). These platinum oxide cluster anions will oxidize CO to C02 and produce [PtJ, which can be reoxidized by N20 or 02. Thus a cyclic catalytic system for the oxidation of CO by N20 or 02 is produced. 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

Bare group 13 metal vertices (e.g., Ga, In, Tl) provide, as noted above, only one skeletal electron each to polyhedral cluster structures. Thus it is not surprising that the bare metal cluster ions Enz (E = group 13 element) found in homonuclear alkali-metal/group 13 intermetallic phases [86-89] (mainly for In and Tl) have charges less negative than the — (n + 2) (i.e., z [Pg.21]

Mingos rules. This hypoelectronicity or electron poverty (fewer than the Wade-Mingos 2n + 2 skeletal electrons) in the bare metal cluster anions Enz (z < n + 2) leads to deltahedra not only different from those in the deltahedral boranes but also different from those in hypoelectronic metal carbonyl clusters of metals such as osmium. 

Abstract This chapter reviews the methods that are useful for understanding the structure and bonding in Zintl ions and related bare post-transition element clusters in approximate historical order. After briefly discussing the Zintl-Klemm model the Wade-Mingos rules and related ideas are discussed. The chapter concludes with a discussion of the jellium model and special methods pertaining to bare metal clusters with interstitial atoms. 

The chemistry of bare metal clusters with interstitial atoms is clearly the new frontier as indicated by two recent serendipitous discoveries ... 

These exciting recent developments are indications that much more new and exciting bare metal cluster chemistry remains to be discovered. 

The mobility of metal atoms in bare metal clusters and small metallic nanoparticles (NPs) is of fundamental importance to cluster science and nanochemistry. Atomic mobility also has significant implications in the reactivity of catalysts in heterogeneous transformation [6]. Surface restmcturing in bimetallic NP and cluster catalysts is particularly relevant because changes in the local environment of a metal atom can alter its chemical activity [7, 8]. 

The facile loss of carbonyl groups in the mass spectra of metal carbonyls permits the generation of novel bare metal cluster ions Mj in the mass spectra of polynuclear metal carbonyls of the type Mx(CO)y. Thus in the mass spectra of Mn2(CO)io and Co2(C0)s all carbonyl groups are lost before rupture of the metal-metal bond resulting in the production of the bimetallic ions Mn and Co 2, respectively 14>. 

All chemical reactions of bare metal clusters in the gas phase that have been studied to date can be characterized as addition reactions. These reactions yield products or adducts that are the result of addition, or addition followed by subsequent elimination, and are the cluster analogs of the chemisorption of molecules onto metal surfaces. The nature of these experiments precludes the detection of gas-phase reaction products that have desorbed from the cluster and to date no example of catalytic chemistry using gas-phase clusters has been reported. 

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