The preparation and characterization of bis-chelated nickel(II) complexes of the 6-methylpyridine-2-carboxaldehyde Schiff bases of S-alkyldithiocarbazates and the X-ray crystal structure of the bis {S-methyl-β-N- (6-methylpyrid-2-yl)- methylenedithiocarbazato}nickel(II) complex