The pentakis(2,4-dimethylphenoxo) niobium (V) complex of composition Nb(OC6H3(CH3)2-2,4)5 has been synthesized by the reaction of niobium pentachloride with 2,4-dimethylphenol in 1:5 M ratio in CCl4 and characterized by elemental analyses, molar conductivity, molecular weight determination, IR, 1H NMR and mass spectral techniques. The geometrical optimization of the complex has been performed by DFT in SIESTA code using standard conjugate gradient (CG) technique. From bond lengths and bond angles of the optimized structure a trigonal-bipyramidal geometry around niobium has been proposed. The HOMO and LUMO energies were analysed to predict the reactivity of the complex and to evaluate global reactivity descriptors-ionization potential (IP), electron affinity (EA), chemical potential (μ), hardness (η), softness (S), electronegativity (χ) and global electrophilicity index (ω). The molecular orbital contributions have been analysed from the total density of states (TDOS), partial density of states (PDOS) and crystal orbital Hamilton populations (COHPs). The thermal behaviour of the complex studied by TG and DTA techniques has indicated it to decompose in single step yielding Nb2O5 as the decomposition product authenticated by IR spectra. The kinetic and thermodynamic parameters from TG data have been computed using Coats–Redfern and Achar equations and F1 first order random nucleation mechanism has been deduced.