Hydrated manganese(II) cyanato complexes of 3- and 4-acetyl, carbomethoxy-, carboethoxy- and benzoylpyridine derivative ligands and crystal structure of trans-dicyanato-bis(4-acetylpyridine)diaquamanganese(II); [Mn(4-acetylpyridine)2(NCO)2(H2O)2]