Centered heteroatomic cluster

S-adenosylmethionine (SAM or AdoMet) superfamily, aconitase, and others), enzymes that contain Fe-S heteroatomic clusters (nitrogenase iron-molybdenum cofactor (FeMoco), carbon monoxide dehydrogenase (CODH), and acetyl CoA synthase (ACS)), and enzymes that contain unique ligation sets around specialized iron centers ([NiFe] and [FeFe] hydrogenases) (Figure 1). ... 

Early interest in heteroatom clusters having alkali metals as the host was academic rather than dictated by precise observations. The main question regarded the extent to which the jellium-derived shell model retained its validity. However, this question was approached on the basis of oversimplified structural models in which the heteroatom (typically a closed-shell alkali-earth such as Mg) was located at the center of the cluster [235, 236]. In this hypothetical scheme, the perturbation of the electronic structure relative to that of the isoelectronic alkali cluster is somewhat trivial for instance, in the Na Mg system the presence of Mg would only alter the sequence of levels of the shell jellium model from Is, Ip, Is, 2s,. .. (appropriate to sodium clusters) to Is, Ip, 2s, Id,. .. (see also [236]). This would lead to the prediction that Na6Mg and NasMg are MNs. 

The vast majority of metal-rich rare earth-metal halides form clusters R that incorporate an atom Z, or a small atom group, in the center of the cluster, hence heteroatomic clusters ZR need to be considered. These clusters are surrounded by ligands, usually haUdo ligands X of the triad chloride, bromide, and iodide. Figure 10 depicts such an isolated cluster complex [ ZR X i2X 6] ... 

Because the radius of an octahedral cavity is 0.414 times the metallic radius of the metal atoms on the surface of the cluster, only the first-row main-group atoms can be incorporated in this site. There are several octahedral carbido- and nitrido-clusters, but examples are also known that incorporate hydrogen and boron. A series of related clusters include [Ru6H(CO)i8] , [Ru6B(CO)i7] , [RueQCO)] ], and [Ru6N(CO)i6] , all of whieh contain a fully encapsulated interstitial heteroatom lodged in the center of the octahedral cavity. 

Sn-R, and Sb Ni(CO)3. Several other closely-related icosa-hedral nickel clusters have been isolated and studied by electrochemistry in the last few years. These include examples of an Ni icosahedron centered on a heteroatom, e.g. [Nii2( tt,2-E)(CO)22] - (E = Ge, Sn) or [Ni,5(/ii2-Sb)(CO)24]"... 

Bonding interactions in these cluster complexes are—as will be pointed out more clearly in Section 5—predominantly of the heteroatomic Z—R and R—X type with only little to no homoatomic Z—Z (only in polymeric stmc-tures possible), R—R, and X X contributions. Thus, the picture usually shown highlighting the octahedral cluster with an endohedral atom in the center ought to be replaced by a picture emphasizing the polar covalent Z—R interactions (see Fig. 5). 

Trigonal prismatic structures are not found very often in simple hexanuclear clusters. Most compounds with this metal atom arrangement are carbides and nitrides in which the heteroatom occupies the center of the prism. 

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