Why can't I define a commutator operator in this way?

I am trying to define some operators I use frequently. For example, I defined a KroneckerProduct operator as $\otimes $ as follows:

a_⊗b_:=a.b-b.a 

And this works fine.

Then I want to define a commutator used in quantum mechanics act like follows:

$${\left[ {a,b} \right]_ - } = a.b - b.a$$ $${\left[ {a,b} \right]_ + } = a.b + b.a$$

where a and b are matrices.

So I naively write the definition in Mathematica as follows:

$${\left[ {a{\rm{\_}},b{\rm{\_}}} \right]_ - }: = a.b - b.a$$ $${\left[ {a{\rm{\_}},b{\rm{\_}}} \right]_ + }: = a.b + b.a$$

But this will not work.

1. How do I define the commutator in this form (appearance)?

2. And why does the definition for a_⊗b_ hold, but the similar definition for commutator [a_, b_] does not hold?

There is another question which actually should create another post. But I choose just to add it here.

If I defined the KroneckerProduct as the operator ⊗ in the way I did, then I found that in parallel computing, I can't simply distribute such a definition of ⊗ like:

DistributeDefinitions[⊗] 

Mathematica doesn't allow me to do that. Of course the Parallelize will not distribute the ⊗ definition automatically either. So if an expression contains a ⊗, simply Parallelize it, the computation will get stuck and no answer will come out.

So my second question is:

How do I distribute such an operator definition in parallel computing?