Would someone be willing to give a formal definition/explanation of hard vs. soft nucleophiles, and hard and soft acids? The things I know (or think I know) already are that hard nucleophiles tend to do 1,2-addition to a,B-unsaturated ketones, while a soft nucleophile would favor a 1,4-addition, no? I would guess that an organolithium would be an example of a hard nucleophile, while an organocuprous reagent (R2CuLi), for example, would be a soft nucleophile.
From other threads I've been reading, I get the feeling that it is something to do with what orbital is going to be forming the new bond, perhaps? In a recent thread on SN2 reactions, it was stated that hydrogen sulfide is a softer nucleophile than water, the difference being the 3p orbital involvement, rather than a 2p, I would guess.
I also know that it has something to do with oxidation and reduction, something to the effect of soft acids are easily oxidized, and soft bases are easily reduced.
Something I am confused on is that I have seen H- called a soft base, and that seems contradictory to the requirements that I was aware of, such as high polarizability to be soft. Same goes for RLi, I have seen it called a soft base as well.