  Using first-principles density functional theory based calculations, we
analyze the structural stability of small clusters of 3$d$ late transition
metals. We consider the relative stability of the two structures - layer-like
structure with hexagonal closed packed stacking and more compact structure of
icosahedral symmetry. We find that the Co clusters show an unusual stability in
hexagonal symmetry compared to the small clusters of other members which are
found to stabilize in icosahedral symmetry based structure. Our study reveals
that this is driven by the interplay between the magnetic energy gain and the
gain in covalency through $s$-$d$ hybridization effect. Although we have
focused our study primarily on clusters of size 19 atoms, we find that this
behavior to be general for clusters having sizes between 15 and 20.
