Should procurement always be involved when negotiating very large and complex agreements?

We're a new company and still putting together to right processes to best manage the business while at the same time experiencing dramatic growth. That growth requires significant construction and engineering services that we contract outside the company. Up to this point, our internal engineers, legal department and the CEO have been involved in the negotiations leaving procurement to simply generate and approve the PO after the deal is all buttoned up. Is this okay? Does this potentially violate any sort of controls that should be in place? Do we expose ourselves to any vulnerabilities?