The structural evolution and bonding of a series of bimetallic cluster of metal cluster anions of niobium with an external aluminum atom, NbnAl- (n = 2-10), are investigated using earlier photoelectron spectroscopy and theoretical calculations. For each niobium aluminum alloy cluster, many low-lying isomers are generated using the Saunders 'Kick' global minimum stochastic search method. Theoretical electron detachment energies are compared with the experimental measurements to verify the ground states of the bimetallic clusters obtained from DFT calculations. The current Letter shows that the dopant atom always occupies the surface of bimetallic clusters and structural transition does not take place until n = 10. (C) 2012 Elsevier B.V. All rights reserved.