Cluster compounds electron-rich clusters

Exceptions among electron rich clusters include compounds in which certain d orbitals are empty, while the s and p orbitals are occupied. This is equivalent to the formation of multiple M — M bonds. Only a few such compounds are presently known, for example, [Re4H4(CO)i2] (56 instead of 60e), and [Os3H2(CO)io] 46e instead of 48 ). The skeletal bonds in those compounds may be represented as follows ... 

The electron rich clusters of the first series of the transition metals are generally more sensitive to oxidation than clusters of the second and third series. The most stable electron-rich clusters are those of groups 8,9, and 10. Clusters of rhodium and iridium are exceptionally stable. However, some clusters of the first series of the transition metals can be stable. This is particularly true for compounds with stable electron configurations containing n-l-2, or n- -3 skeletal electron pairs. For example,... 

The discussion so far has concentrated on the structural chemistry of the electron rich clusters of the [MgXg] type. To prepare for the next paragraph, interest is now focussed on metals with smaller numbers of valence electrons which then prefer to form [MgX j type clusters. A tremendous amount of work has been performed on [MgX,2] type halide clusters of the metals Nb and Th both in the solid state and in solution. Ousters with 14, 15, and 16 electrons in M-M bonding states are well known. Examples of compounds with this cluster type but formed with divalent anions had been very scarce, being represented by only two which contained the [NbgOi2] cluster. [57, 58] Recently, a rather broad chemistry of reduced oxoniobates based on this cluster has emerged. [16]... 

Ligand Preferences. The LVC s are electron rich and in order to exist in stable compounds they require ligands, such as CO, that are weak donors and good tt acceptors. On the other hand the HVC clusters are not attractive to such ligands nor do they require them for stability. In this respect, there is a direct parallel to mononuclear compounds where M° prefers CO, RNC or similarly ir-acidic ligands while the Mn+ (n = 2-4) ions do not generally form CO complexes. 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

The electron-rich trinudear Au3 (bzim) 3 and Au3 (carb) 3 react with F6 to yield the sandwich clusters [n Au3(bzim)3 2](PF6) 29 and [n Au3(carb)3 2](PF6) 30 [54]. The cation of both compounds contains atom which interacts in a distorted trigonal prismatic coordination with six Au(I) atoms from two cyclic Au3C3N3 moieties at Au- distances ranging from 2.971(1) to 3.107(1) A, indicative of appretiable metal-metal interaction. Two Au(I) atoms on each planar trinudear unit are involved in intermolecular aurophilic bonding interactions [Au-Au 3.109(1)/3.066(1) A in 29 3.059(1)/3.052(1) A in 30], thus resulting in an infinite columnar chain with a BBABBA- pattern (Figure 4.11). 

Our knowledge concerning soluble metal complexes with sulfide ions as ligands has increased considerably during the last two decades and this kind of Compound is still of topical interest. Some of the reasons for this are the development of a very flexible and fascinating structural chemistry of multinuclear metal-sulfur complexes, the fact that the active sites of some electron transfer proteins contain metal ions and labile sulfur,41,42 and also the relation of metal-sulfur cluster compounds to some heterogeneous catalysts. In addition, apart from the numerous binary and ternary sulfides which occur in nature, we have at our disposal a rich solid state chemistry of metal sulfides, which has been reviewed elsewhere and will be excluded here.43"17... 

Electrochemical investigations of [Au2Re2(H)6(PPh3)6](PF6) have shown that this cluster displays a rich redox chemistry (244) and it has been concluded that the existence of this reversible electron transfer chemistry is due to the considerable amount of Re-Re multiple bond character that is retained in the cluster. A number of gold-osmium cluster compounds have been reported to show reversible redox chemistry (245). 

The chemistry of reduced Nb and Ta hahdes is rich in clusters with various structures. The metal atoms assemble with metal metal distances close to those in the metal into triangular and tetranuclear clusters but the dominant structural motif is that of the octahedral M6X12 and NbeIg types. Binary, ternary, and quaternary compounds aU crystallize in that type. The Me clusters are characteristic of the chemistry of the lower oxidation states of Nb and Ta, although not restricted to them. These electron-deficient clusters are based on metal ions with average oxidation numbers between III and I. 

Qualitative molecular-orbital analysis of computational results. Rather than employing purely qualitative, symmetry-based theory, we can also perform calculations on solids or clusters and then analyze them using qualitative MO arguments. We will consider two such studies (1) an EHMO study of SiOj in (3-quartz, stishovite, and hypothetical silica-w structures (Burdett and Caneva, 1985), and (2) an MS-SCF-Aa study of electron-rich transition-metal compounds (Tossell and Vaughan, 1981). 

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