Bare metal cluster anions

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. 

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. 

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 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. 

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. 

The efforts to rationalize the formulas and structures of Zintl ions and related species predated extensive definitive structural information on anionic post-transition metal clusters obtained by Corbett and his group in the 1970s [8, 9]. After enough such structural information on the bare post-transition metal clusters became available, the resemblance of their polyhedra to the known polyhedral boranes became apparent. For this reason, the simple Zintl-Klemm concept has been largely superseded by newer, more advanced descriptions of chemical bonding in such clusters, initially those applied to the polyhedral boranes. 

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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