Transition-metal Schiff-base complexes as ligands in tin chemistry Part 6. Reactions of diorganotin(IV) dinitrates with M(3MeO-sal1,3pn) [M = Ni, Co or Zn; H23MeO-sal1,3pn = N,N′-bis(3-methoxysalicylidene)-propane-1,3-diamine]