Shareholder Revolt Snaps Into Deal-Killer Mode: Scrapping of the VNU/IMS and Mylan/King deals warns boards that they may have to finesse angry stockholders when seeking deals.

Dealmakers have a new obstacle to overcome as they try to close transactions: Activist shareholders are using their clout not only to force sell-offs, spin-offs, and other restructurings but to sink entire deals.

Shareholder activists generally are playing a more assertive role in determining the outcome of public companies, and the intensification of their pressures on managers and directors offers potential pitfalls for executives but also provides opportunities for dealmakers.

Several deals have been abandoned because of shareholder opposition. These include the proposed acquisition of IMS Health Inc. by Netherlands-based research giant VNU; the ill-fated merger of King Pharmaceuticals Inc. and Mylan Laboratories Inc.; and the cross-border linkage of trading markets Deutsche Borse and the London Stock Exchange. In addition, Sovereign Bancorp's proposed acquisition of Independence Community Bank along with Sovereign's plan …