Cluster compounds interatomic distances

This disilaborane was an unexpected co-product in the synthesis of decaborane-alkylamine polymers. The Si2Bio cluster core consists of a distorted icosahedron in which the two silicon atoms occupy adjacent positions. The Si-Si interatomic distance is 2.308(2) A, which is slightly less than the Si-Si distance in organodisilanes (2.35 A) and the Si-B distances [2.017(3) to 2.116(3) A] are very close to the sum of the covalent radii of the two atoms (2.07 A). Further derivatives with disilaborane cluster geometry are known for the phenyl substituted compounds l,2-Ph2-doso-l,2-Si2B1oH1o and l-Me-2-Ph-doso-l,2-Si2BioHi0 [6, 7]. In addition to these disila-doso-dodecaborane clusters one example with two different group 14 elements as a part of the cluster core is known. In Scheme 3.3-2 the synthesis of this sila-stanna-doso-dodecaborate is shown. The structure of this heteroborate was determined in the solid state and the Si-Sn distance is 2.608(4) A (Scheme 3.3-2) [8]. 

In order to make these points clearer, interatomic distances in representative compounds are listed in Table VII. The distortion of Mo6 should be caused by electronic effects from the individual cluster units and/or steric effects due to the linkage of the clusters. At one time the electronic effects were considered more important based on the band calculations (58, 84). The strong eg Mo rf-bonding character of the conduction band causes contraction of the Mo octahedron, and the filling of the tlu(Oh) - eu (S6) and t2u (Oh) —> au (S6) band is related to the smaller intratriangle distance. Namely, the larger shift of electrons from Mx and chalcogen atoms to the molybdenum atoms is related to smaller distortion of the Mo6 octahedra. 

The assessment of for the main isomer of IV (IVa) has been exploited to yield the H—H interatomic distances in the two minor isomeric forms of this compound (IVb and IVc, see scheme 3). In fact, a careful examination of the hydride region in the H NMR spectrum of IV at 183 K (fig. 1) reveals the presence of low intensity resonances ca 1 100 with respect to the main signals) ascribed to the occurrence of two novel structural isomers of the cluster. Since... 

A comparison of the short Nb-Nb interatomic distances in compounds with condensed clusters is given in Table 5. There seems to be a correlation between the number of valence electrons available per two-electron three-center bond and the average Nb-Nb bond length in the NbeOn clusters (Fig. 21). The average Nb-Nb bond length is 287 pm in BaNbsOs with 1.37 electrons per three-center bond whereas t Nb- Nb = 297 pm in NbO with 1.13 electrons. It must, however, be remembered that in all compounds there is a balance between A-O, Nb-O, 0-0, and Nb-Nb distances. The two types of niobium atom in the condensed clusters give rise to three kinds of Nb-Nb bonding Nb -Nb Nb " -Nb and Nbi l-Nb J and as a result the Nbe octahedra are always distorted. The... 

Table 5. Nb-Nb interatomic distances (in pm) in compounds with condensed clusters. Nb " denotes niobium coordinated with 4 oxygen atoms, Nbf- denotes niobium coordinated with 5 oxygen atoms, and Nb f denotes niobium coordinated with 6 oxygen atoms. 

TABLE 2 A Compilation of Cluster-Based Electron (CBE) per One Formula Unit and Important Interatomic Distances, in pm, for ZRg Xi2R-Type Compounds ... 

---