Optimized structures of nickel cluster ions, Ni-n(+) (n = 2-8), were obtained by use of the density functional method. In comparison with the reaction of a methanol molecule with Ni-n(+) (chemisorption, demethanation and carbide formation), it is revealed that (1) the total reaction (mainly chemisorption) cross section is anti-correlated to the HOMO-LUMO gap of Ni-n(+) and (2) the selectivity of the carbide formation against the demethanation is determined by a propensity, "d-vacancy" (the number of the d-holes per atom); the demethanation proceeds selectively on Ni-n(+) With a d-vacancy of less than about 1.1 and the carbide formation otherwise.