With the Departure of John Thain from Bank of America, now Merrill Lynch’s former no. 1 and no. 2 guys are gone from the merged entity. Word is that the rest of the former Merrill staffers are now left wondering, as the Wall Street Journal put it, about “a rocky integration into a bank that was supposed to offer stability.”

Is anyone surprised? Corporate marriages are never easy, especially when they’re a product of shotgun weddings. In real life, the key to marriage is compatibility, and successful corporate mergers are no different. That’s why so many of them fail. The suitors get romanced by the idea that one plus one will equal three in terms of market opportunity and back office synergy. But Merrill and BofA differ from culture to compensation, and it may turn out that in this case one plus one may not even equal two.

Our research shows that despite outward appearances, companies often don’t get what they want out of acquisitions. They too often fall for the promise of what could be rather than the reality of what is.

THIS JUST IN: Pfizer is considering an acquisition of Wyeth in a $60 billion deal. Both have been struggling with competition, changing industry dynamics and weak new product pipelines (see “What Not To Cut” below). It will be interesting to watch as things develop whether this is a marriage for love or money.