  We report the structural, magnetic, transport and electronic properties of Nb
substituted LaCo$_{1-x}$Nb$_x$O$_3$ ($x =$ 0--0.2). The Rietveld analysis of
x-ray diffraction data demonstrate structural phase transitions from
rhombohedral to orthorhombic and further to monoclinic with increasing the Nb
concentration up to $x \ge$ 0.2. Interestingly, we observed dramatic changes in
the magnetization (M) with increasing the Nb concentration, as the M sharply
increases below 10~K even at 2.5\% substitution. Furthermore, ac susceptibility
data show the spin-glass behavior in $x=$ 0.1 sample. We find that the density
of states near the Fermi level decreases and the activation energy increases,
which results in the decreasing conductivity with higher Nb concentration. A
significant shift in the peak position of A$_{2g}$ phonon mode has been
observed using Raman spectroscopy, which indicates the change in the coupling
due to the structural distortion with Nb substitution. The core-level
photoemission study confirms that the Nb is present in 5+ valence state. Our
study reveals that the nonmagnetic Nb$^{5+}$ ($d^0$) substitution converts
Co$^{3+}$ ions to Co$^{2+}$ and stabilize both in the high-spin state. Our
results suggest that structural and spin-state transitions as well as the
difference in the ionic radii between Nb$^{5+}$ and Co$^{3+}$ are playing an
important role in tuning the physical properties.
