BIMETALLIC ALKOXIDES OF NIOBIUM

The formation of the I-III group metal alkoxoniobates has been studied using the physicochemical analysis method, namely the investigation o f solubility isotherms for the M(OR)(n)-Nb(OR)(5)-ROH systems (M = Li, Ba, Sc, La, R = 2Et, Pr-i). The crystallization of the following comp lexes has been observed: LiNb(OEt)(6) (I), [LiNbO(OEt)(4)(EtOH)](2) (I I), [BaNbO(OEt)(5)(EtOH)(1,5)](4) (III), BaNb2(OPri)(12)((PrOH)-O-i)(2 ) (IV) and LaNb2(OPri)(13) (V). It has been found that the bimetallic isopropoxide of Sc and Mb does not exist in the solid state. The oxo c omplexes Il and III are formed on storage (or more rapidly on refluxin g) of the solutions of non-ore compounds in alcohols in the presence o f dry air and are, in fact, the products of these compounds oxidation by oxygen. The structure of II is built of dimeric molecules with the metal-oxygen core analogous to that of Ti-4(OMe)(16). The molecule of III is constructed around the [Ba4O4] cubane-like unit, whose oxygen v ertices are occupied by the oxo atoms of NbO(OEt)(5) octahedra. The st ructures of the molecules of IV, V and recently studied Nb(OPri)(5) ar e based on the common motif featuring the M(2)O(2) four-membered cycle s in edge sharing octahedra. All the observed [NbMNb] units are nonlin ear, the molecule of V being asymmetric, for the central [LaO6] octahe dron is sharing simultaneously an edge with one [NbO6] octahedron and a face with the other. The conditions leading to the formation of oxo groups and the influence of the latter on the complexation process hav e been discussed. The literature data on the existence of 3d transitio n metals alkoxoniobates have not been confirmed. It has been found tha t interaction of Nb and Co or Ni isopropoxides takes place only due to preliminary micro-hydrolysis or pyrolysis and leads to formation of o xoalkoxoniobates.