Of all the ways to describe mergers and acquisitions in 2015, “plain vanilla” is not one of them.

Last year was one for the record books, generating $4.7 trillion in announced deals, with the highest percentage ever derived from those valued at more than $5 billion, according to data compiled by Thomson Reuters.

Unlike boom years past, however, this one was characterized by dozens of complex and creative financial structures. Drug makers Pfizer and Allergan sought to combine — effectively through a tax-inversion deal, lowering Pfizer’s tax rate. Dow Chemical and DuPont, two storied chemical giants, agreed to merge into a $130 billion behemoth with the explicit intent to be separated into three companies in a few years.

Other deals, such as Anheuser-Busch InBev’s $104 billion acquisition of rival brewer SABMiller required multiple divestitures before any chance of regulatory approval. Rare financial instruments such as tracking stocks and contingent value rights, both creative ways for adding value for the seller, also appeared in takeovers.

Boardrooms were flooded with so much confidence that executives were willing to stretch beyond the textbook-style acquisition — Company A buys Company B — to get deals done. Financing was cheap because of low interest rates and growth was generally slow, so colossal companies sought purchases as a way to expand.