Niobium interatomic distance

The niobium atom has a slightly distorted octahedral coordination. Interatomic distances between the niobium atom and the two oxygen atoms in trans positions, O-Nb-O are 1.81 and 2.14 A. The niobium atom is shifted from the base plane of the octahedron by 0.23 A, and this shift, in adjacent chains, is in opposite directions. Pakhomov and Kaidalova [204] concluded that the shorter Nb-O bond (1.81 A) is an intermediate between a single and double bond. 

Typical examples of compounds that form chain-type structures are simple pentafluorides of tantalum and niobium, TaFs and Nbfs, which were investigated by Edwards [199]. The cry stal structure of NbF< consists of rings made up of four NbF6 octahedrons that are linked by fluorine ligands that are each shared by two neighboring octahedrons, as shown in Pig. 29. The main interatomic distances (in A) are as follows Nb-Nb 5.80, 3.90 and 4.13 Nh-P,(Ki -. 1.75 and 1.78 Nb-Ftal% - 2.06 and 2.07 angle Nl>F[jf d,e-Nh - 182.5°. 

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. 

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