[that_guy_iain]

Any agreement not to hire employees without including the employees in the negoation would be illegal therefore void in most countries. Including the US where Apple and Google (maybe someone else) were caught having agreements not to hire from each other. It removes the ability for the employee to find the best salary/option/etc for themselves.

[detaro]

There is a difference between overall no-hire agreements (which indeed won't stand in many places) and specific clauses covering only people involved in a common project (and that often don't strictly prevent hiring, but e.g. require compensation).

[that_guy_iain]

I would suspect the same thing applies. Two companies are making agreements that affect a third parties ability to freely market their services. Non-compete clauses in employee contracts are in many countries not legally binding because they affect the employees ability to freely market their services. And in some countries where they are valid, they're are often very strict requirements on what can be put in the clause. I've literally had multiple contracts with non-compete clauses, not a single one was valid.

Clauses such as "If you hire this person you owe us X for an introduction fee" however would be more likely to be valid.