**2.4 Mo+6 oxides**

Bulk polymolybdate chains typically comprise MoO6 coordinated units that are different from the chains of polyvanadate as well as polychromate and these chains are respectively possessed with VO4 and CrO4 groups. This reveals the liking of molybdates for greater coordination groups in comparison with vanadates and chromates units in the respective polymeric structures. Yet, certain exemptions occur to this tendency in the structural chemistry of bulk molybdate. Short coordinated molybdates exist in the dimer of MoO4 which is MgMo2O7 and in the chain of interchanging MoO4 and MoO6 units which are NaMo2O7. Coordination of isolated MoO4 is still somewhat mutual for ortho-molybdates as an example MgMoO4, CuMoO4, Na2MoO4, MnMoO4, K2MoO4, and CaMoO4. Extremely misleading coordination of isolated MoO4 is discovered in Gd2(MoO4)3, Fe2(MoO4)3, Cr2(MoO4)3, and Al2(MoO4)3. Whereas, extremely misleading units of MoO5 are existing in Bi2(MoO4)3 [20, 30]. Further, clusters of polymolybdate are constituted with 6 to 8 MoO6; whereas coordinated units are also recognized for example (NH4)4Mo8O26, (NH4)6Mo7O24, and [NH3P3(NMe2)6]2Mo6O19. Bulk MoO3 (alpha) is comprised of a 3D structure prepared up of extremely misleading units of MoO6. The great misleading existing in bulk MoO3 (alpha) origins the sixth O atom to be positioned extremely distant respected to Mo and, therefore, the structure of the relevant bulk MoO3 (alpha) is well pronounced as comprising of MoO5 units. The bulk MoO3 (beta) crystalline period is one more MoO3 3D structure fabricated up of minute misleading of MoO6 units [31]. Numerous gas-phase mon-oxo molybdenum oxyhalides (X4Mo = O) are also recognized, structural analysis illustrated in **Figure 3**. The Mo = O vibrations fluctuate in the range 1008–1039 cm−1 with the increment in electronegativity of halide in the order Cl *<* F. The gas-phase di-oxo Br2Mo(=O)2 growths to the bands at 995 (*v*s) as well as 970 (*v*as) cm−1 owing to an order of electronegativity as Br *<* Cl *<* F. Therefore, the structure of molybdenum oxides with respect to its inorganic chemistry involves the coordinated units as MoO4, MoO5, and MoO6, with a first choice in polymolybdates for MoO6 latter [32].

For non-SiO2 supported MoOx catalysts, MoOx coordination for the dehydrated surface is subjected to the exposure of surface molybdena and particular oxide support. At faint surface coverage of molybdena that ranges from 5–15% of a monolayer, mainly surface coordinated groups of MoO4 are exist on Al2O3 and TiO2. The parallel Raman spectrum of the above-discussed catalysts also agrees to the extent of minute surface coverage. In addition, MoO4 surface species are correspondingly isolated on both oxide supports. This phenomenon is also authenticated through UV–vis spectra that display huge bandgap energy related to isolated classes. Species with monolayer owns the surface exposure of molybdena, sustained MoO3/TiO2 was establishing to hold MoO6 coordinated groups, and sustained MoO3/Al2O3 was set up to retain a combination of MoO4 as well as MoO6 coordinated species. For monolayer MoO3/Al2O3, the supplementary occurrence of surface MoO6 was also revealed in minor bandgap value of this catalyst. Therefore, UV–vis analysis and Raman measurements for samples discussed above (dehydrated MoO3/ZrO2 and MoO/Al2O3) were quite alike and recommend the similar surface species such as MoOx occur on the supports together by a certain surface exposure. Measurements are taken from Raman approach also discloses characteristics of linking Mo–O–Mo bonds existing in polymolybdates [33–35].
