  Structures, physical and chemical properties of V doped Co$_{13}$ clusters
have been studied in detail using density functional theory based
first-principles method. We have found anomalous variation in stability of the
doped clusters with increasing V concentration, which has been nicely
demonstrated in terms of energetics and electronic properties of the clusters.
Our study explains the nonmonotonic variation in reactivity of Co$_{13-m}$V$_m$
clusters towards H$_2$ molecules as reported experimentally [J. Phys. Chem.
{\bf 94}, 2744 (1990)]. Moreover, it provides useful insight into the cluster
geometry and chemically active sites on the cluster surface, which can help to
design better catalytic processes.
