Cobalt reacts with a sodium hydroxide solution to form a complex that has no charge. The hydroxide ions remove the hydrogen ion from the cobalt, leading to this reaction.
Cobalt is a fairly reactive element. Unless the cobalt is in a powder form, it will not burn when it combines with oxygen from the air. It also will react with many acids to produce a hydrogen gas. Cobalt will not react with water that is at room temperature. Another reaction that occurs with Cobalt is with an ammonia solution. During this reaction, the hydrogen atoms are removed from the hexaaquacobalt ion to produce a neutral complex. This will dissolve with the addition of excess ammonia solution. In this instance, the ammonia solution acts as both a ligand and a base. The colors of this mixture change rapidly during the reaction.