Some nickel(II) β-amino-ketone and β-amino-imine compounds; the structures of mer-bis(7-amino-4,4-dimethyl-5-azaheptan-2-one) nickel(II) perchlorate,(2,7,9,9-tetramethyl-3,6,10-triazadodeca-2,6-diene-12-amine)nickel(II) tetrachlorozincate water and (4,6,6-trimethyl-3,7-diazanon-3-ene-1,9-diamine)(ni