The reaction of [Ni(tn) 3 ] 2+ (tn = trimethylenediamine) with carbohydrates: X-ray crystal structure of bis{1-[(3-aminopropyl)-amino]-1,6-dideoxy- L -mannose}nickel( II ) dibromide dihydrate methanol solvate: {[Ni( L -rham–tn) 2 ]Br 2 ·2H 2 O·CH 3 OH (rham = rhamnose)}