Mass spectrometric study of polydentate Schiff base coordination complexes. Cobalt(II), nickel(II), and copper(II) complexes of N,N'-bis(salicylidene)heptenediamine, -3,3'-bis(aminopropyl)amine, -3,3'-bis(aminopropyl) ether, and -3,3'-bis(aminopropyl) sulfide