Cluster complexes, valence

For trinuclear cluster complexes, open (chain) or closed (cycHc) stmctures are possible. Which cluster depends for the most part on the number of valence electrons, 50 in the former and 48 in the latter. The 48-valence electron complex Os2(CO)22 is observed in the cycHc stmcture (7). The molecule possesses a triangular arrangement of osmium atoms with four terminal CO ligands coordinated in a i j -octahedral array about each osmium atom. The molecule Ru (00) 2 is also cycHc and is isomorphous with the osmium analogue. 

For tetranuclear cluster complexes, three stmcture types are observed tetrahedral open tetrahedral (butterfly) or square planar, for typical total valence electron counts of 60, 62, and 64, respectively. The earliest tetracarbonyl cluster complexes known were Co4(CO)22, and the rhodium and iridium analogues. The... 

L = axial ligands, n = 0, 1, 2) have been extensively investigated regarding their specific electronic, chemical, and physical properties [4], Particularly, oxo-centered triruthenium cluster complexes with bridging acetates attracted the most attention owing to their synthetic accessibility, multiple redox behavior, intriguing mixed-valence chemistry, and versatile catalytic properties [5-7]. 

The chemistry of cluster complexes, e.g. of the sort [FeitSi, (SR) i,] 2, is of particular interest since such complexes are known to be close representations or synthetic analogues of the redox centres present in various iron-sulphur proteins. It is important to know whether the valence electrons are localized or delocalized in such complexes - in fact several studies by e.s.r., n.m.r., and, more recently, resonance Raman spectroscopy have shown that such clusters are delocalized rather than trapped-valence species. This result is linked with the most important biophysical property of iron-sulphur proteins, viz. that of electron transfer. Rapid electron transfer is possible if any consequential geometric rearrangements around the metal atom sites are small, as implied by many resonance Raman results on such cluster complexes (cf. the small-displacement approximation, which provides a basis for enhancement to fundamental but not to overtone bands) (22). Initial studies of [MSi,]2- ions (M = Mo or W) (23,24) have since been supplemented by studies of dinuclear species e.g. [(PhS)2FeS2MS2]2 (25) and cluster species... 

As discussed in section 2.8, relativistic effects on the valence electronic structure of atoms are dominated by spin-orbit splitting of (nl) states into (nlj) subshells, and stabilization of s- and p-states relative to d- and f-states. Here we examine consequences of relativistic interactions for cubo-octahedral metal-cluster complexes of the type [M6X8Xfi] where M=Mo, Nb, W and X=halogen, which have a well defined solution chemistry, and are building blocks for many interesting crystal structures. 

With a valence electron (VE) count of 46, the complexes 5 are two electrons short of the magic number required by the Wade-Mingos rules for an electronically saturated M3C cluster complex. It is therefore not surprising that these cluster complexes readily react with small inorganic and organic molecules to give elec-... 

A number of electron counting schemes have been developed to rationalize or even predict the metal framework geometries adopted by metal cores in cluster complexes. The simplest of these is the Effective Atomic Number (E.A.N.) rule which is based on the 18 electron rule , and assumes that each metal uses its five d, three p and one s valence orbitals for bonding, and that all metal-metal bonds are two centre - two electron bonds. Clusters which obey this rule are said to be electron precise . However, often there may be a number of different structures, with the same electron count, which conform to these rules and there is no discrimination... 

Cyclic voltammetry demonstrated that the pyrazine-bridged dimer Ru3(pyz)Ru3 has an extensive oxidation state chemistry, and that the cluster-cluster mixed-valence ions [Ru3(pyz)Ru3] and [Ru3(pyz)Ru3] are discrete species in solution. Ruthenium ESCA data for the perchlorate salt of the +1 ion reveal the presence of distinct cluster units, Ru3 (pyz)Ru3 and therefore indicate that intercluster interactions across pyrazine are weak. However, the electrochemical data reveal that the extent of cluster-cluster interaction increases as the electron content of the system increases. The synthetic chemistry here is very promising, and we should be able to prepare complex two-dimensional systems including polymers and to investigate cluster-cluster and metal ion-cluster interactions. 

The duster molecule has a diameter of about 3000 pm and the Ni-Ni distances in 51 lie in the range 236-312 pm. A description of the bonding in terms of the 18e rule is therefore not possible. 51 contains 448 valence electrons, which is in good agreement with the number predicted for M clusters (n > 13) on the basis of the Hume-Rothery rules for intermetallic phases (440-444e"). [26-29] This indicates that the properties of cluster complexes will approach those of metals as the number of metal atoms increases. It should be emphasized, however, that the structure of the Ni34 unit cannot be described in terms of those prindples derived for metals and intermetallic phases. 

The rare-earth elements, R, may form clusters of six to eight R atoms encapsulating an endohedral transition metal atom T to compensate for the low number of valence electrons of the R atoms (only three). These heteroatomic clusters, TR,, are surrounded by halide ligands, X, to build cluster complexes, TR ... 

High-Valence Metal Clusters. Structural properties of selected hexanuclear high-valence cluster complexes are described in Table 2.5. A series of molybdenum and tantalum derivatives of type [(M6Y8)X6] and [(M6Ys)L8] (M = Mo or W X = halide or alkoxide Y = halide or other monovalent anion and L = neutral Lewis-base) are known. The structures of these species are like that of the anion [(Mo6Cl8)Cl6] illustrated in Fig. 2.19 in which the molybdenum atoms are in the vertex of an octahedron. Metal-metal distances of about... 

Using high-valence oxometalates as ancillary components, heterometaUic cluster complexes can be formed by a one-step synthesis. The solvothermal reaction of Co(OAc)2, TC4A, and Na2M 04 in EtOH/CHC readily afforded... 

A feature of transition metal cluster complexes is that they can undergo electron transfer reactions to give mixed-valence clusters. A second feature of these complexes is that, in addition to homometallic clusters, heterometallic cluster complexes can be prepared that have two or more different metals within the cluster core. Examples of such complexes for which emission has been observed are (DENC)3Cu 2Cu°Co(NS)2CU and (DENC)3Cu3M(NS)CU [DENC = iV,A -diethyl-nicotinamide NS = 5-methylhydrazine-carbodithioate Schiff base M = Co(II), Ni(II), Cu(n), Zn(n)]. For these complexes the emission maxima and lifetimes are shown in Table The emission from all the complexes is centered on the... 

Syntheses, crystallization, structural identification, and chemical characterization of high nuclearity clusters can be exceedingly difficult. Usually, several different clusters are formed in any given synthetic procedure, and each compound must be extracted and identified. The problem may be compounded by the instabiUty of a particular molecule. In 1962 the stmcture of the first high nuclearity carbide complex formulated as Fe (CO) C [11087-47-1] was characterized (40,41) see stmcture (12). This complex was originally prepared in an extremely low yield of 0.5%. This molecule was the first carbide complex isolated and became the foremnner of a whole family of carbide complexes of square pyramidal stmcture and a total of 74-valence electrons (see also Carbides, survey). 

P4) is closely similar with P-P distances of 216 pm (smaller than for P4 itself, 221pm). Indeed, a whole series of complexes has now been established with the same structure-motif and differing only in the number of valency electrons in the cluster some of these are summarized in Table 13.11. The number of valence electrons in all these complexes falls in the range 30-34 as predicted by R. Hoffmann and his colleagues.Many other cluster types incorporating differing numbers of Group 15 and transition metal atoms are now known and have been fully reviewed. ... 

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