In this paper, we evaluate the capacity region of wireless ad hoc networks under a recently proposed space-time collaborative scheme. This protocol allows idle nodes to cooperate with the source opportunistically, i.e., whenever their wireless channel from the source is advantageous. The scheme is intrinsically distributed and well suited for an ad hoc scenario. It is shown analytically and through simulation that the capacity region of opportunistic collaboration is equal to or larger than (centralized) optimal multi-hop in case spatial reuse is not allowed by the transmission protocol. On the other hand, in case spatial reuse is possible, the relation between the two capacity regions has to be studied case by case. Simulation results prove that opportunistic collaborative communication is a promising paradigm for ad hoc networks that deserves further investigation.