reach/complete/close a deal

Do you mean the issue is not predictable, or that we can't predict when it will happen?

I want to say that negotiation is actually unpredictable and frequent event-or that it is not anymore predictable or rare.

Sorry, I don't get you... Anyway, you need a "an" : negotiation is an unpredictable etc event"

We can't predict what will happen

Negotiation in companies/Company negotiation is an often venturous and relatively rare event by which we try to reach/close/complete? some special business deal or tightly related organizational interest.

That's about the extend of the help I can offer here, I'm a very unskilled businesswoman!

Company negotiations are rather/relatively rare/seldom, and often venturous; they aim at striking some special business deal, or at reaching a tightly related organizational interest.

Yet, I don't quite understand what it means

It is a kind of a definition of negotiation: Negotiation is X and Y activity whose aim is to A and B.

p.s. I

Just another version:

Negotiation in a commercial context is an unpredictable and relatively rare event by which we attempt to clinch a particular business deal or advance the commercial interests of the organization in some way.

MrP

Thanks Mr P. So, you would completely omit '' tightly related interests''?

Well, we can't really reach/close/complete an interest. It's difficult to get the sense of the original; but I take 'tightly related' to mean 'closely related', and 'organizational interest' to mean 'thing that is in the interests of the organization'. However, it's not at all clear what the latter could be 'closely related' to. So I've used a more general phrase to cover all the possibilities.