There are many examples in all of the Mistborn novels of Allomancers using brass and zinc to Soothe and Riot emotions. Most of the time, these emotions are clearly definable as emotions: hope, anger, despair, etc. However, occasionally Allomancers are shown to affect more esoteric things: Vin is shown Rioting curiosity and boredom when
infiltrating the Fadrex cache.
Is there any clear definition of what counts as an "emotion" that can be affected by Allomancy? Is there an example of an Allomancer trying to Soothe an emotion and failing, because that emotion isn't a valid 'target'?