Cluster fragmented

A fascinating variety of discrete (or occasionally polymeric) polynuclear halogeno complexes of As, Sb and Bi have recently been characterized. A detailed discussion would be inappropriate here, but structural motifs include face-shared and edge-shared distorted (MXe) octahedral units fused into cubane-like and other related clusters or cluster fragments. Examples (see also preceding paragraph) are ... 

Mercury has a marked ability to bond to other metals. In addition to the amalgams aheady mentioned (p. 1206) it acts as a versatile structural building block by forming Hg-M bonds with cluster fragments of various types e.g. reduction... 

Cluster Metsd ion Cluster fragment Ground state Ref. 

Small Co metal clusters Cora (ra = 2-8) react with CO, with sequential addition leading to the saturated Co species [Co2(CO)7], [Co3(CO)i0], [Co4(CO)i2], [Co5(CO)13] and [Co6(CO)i5]-.71 This points towards one of the features of low-valent Co carbonyls a tendency to form stable clusters. Reactivity of Co with 02 is higher but leads to cluster fragmentation, whereas N2 is essentially unreactive. Entry into carbonyl chemistry of low-valent Co is frequently via the well-known dimer Co2(CO)8. A range of reactions commencing with this compound has been developed, as follows. 

Compounds (I), (II), (III), and (IV) (Table 2) have similar crystal and molecular structures (Fig. le), characterized by the following the crystals of these compounds are made of octanuclear cluster fragments [Tc8Br4(/(-Br8]"+... 

Thus, the investigations performed have shown that cluster fragments in technetium compounds are thermally rather stable. However, in a number of compounds thermal decomposition occurs at comparatively low temperatures, which is mainly related to the instability of organic cations and the weak bonding of technetium with organic ligands. 

Lastly, it is appropriate to comment on the relationships between the intermediates seen in photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. Earlier kinetics studies (] 3) of the reactions of Ru3(CO)i2 with various phosphorous ligands PR3 have found evidence for both first order and second order pathways leading to substitution plus some cluster fragmentation. The first order path was proposed to proceed via reversible CO dissociation to give an intermediate analogous to II. 

No cluster fragmentation was observed initially, although long term photolysis (313 nm) of B in CO saturated cyclohexane eventually did lead to fragmentation to Ru(C0)5 plus acetaldehyde ... 

Phosphine-substituted complexes have shown promise for cluster catalysis, especially in the case of chelating ligands, because of the added stability that might help avoid cluster fragmentation. Bergounhou et al. reported a detailed study of the hy-... 

Cluster fragments, e.g., M(CO)3, can display a variety of bonding capabilities even within the same molecule. This is the case in the structures of the binary carbonyls of osmium. The simplest cluster, illustrated in (a), is Os3(CO)12. This consists of an equilateral triangle... 

Substitution of CO by larger ligands such as tertiary phosphines will tend to give either coordinatively unsaturated clusters or cluster fragmentation. 

These ideas are not restricted simply to binary carbonyls and may apply equally to all conventional cluster fragments. On hydrogenation, Os3(CO)12 reacts first to produce H2Osa(CO),0 and then (at slightly higher temperatures) t Os COl. In this case, the polymerization step H2Os3(CO)10 — H4Os4(CO)i2 may be viewed as... 

Carbonyl dissociation will diminish in the presence of external CO. Carbon monoxide will also tend to trap out the unsaturated low-cluster fragments and therefore prevent cluster formation. The formation of Os5, Os8, Os7, and Os8 clusters from Os3(CO)12 is [CO]-dependent, the relative yields of the lower or higher clusters depending on the CO pressure. 

FIGURE 3.10. Reductive cleavage of aliphatic polychloroalakanes in a nonprotic medium. Dp is the interaction energy between clustered fragments. 

M. Farnik, M. Weimann, C. Steinbach, U. Buck, N. Borho, T. B. Adler, and M. A. Suhm, Size selected methyl lactate clusters Fragmentation and spectroscopic fingerprints of chiral recognition. Phys. Chem. Chem. Phys. 8, 1148 1158 (2006). 

The scheme of cluster condensation or cluster fragment condensation leads eventually to structures observed in bulk metals. Particularly through extensive condensation of tetrahedral and octahedral clusters, arrangements closely related to the hexagonal and cubic close-packed structures can be obtained. Condensation also of icosahedral five-fold symmetrical clusters may be related to crystalline and quasicrystalline metallic structures. 

Valuable findings on the electronic ground and excited states of clusters have been derived from laser-induced multi-photon ionization (MPl) investigations, such as laser-induced fluorescence (LIF) and REMPI. This latter technique is particularly promising since it enables mass selection of cluster species and their spectral and thermochemical characterization. The complex is excited from its electronic ground state from a photon and then ionized by a second photon of equal or different frequency, near threshold to avoid cluster fragmentation. ... 

In general, sources of metal cluster fragments, such as Ru3(CO)io(NCMe)2 or Ru3(/i-dppm)(CO)io react readily with 1,3-diynes or poly-ynes initially to give alkyne complexes, which readily undergo further reactions as a result of activation by the cluster core. 

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