An attorney for the Williams Cos. on Wednesday asked the Delaware Supreme Court to rule that Energy Transfer Equity breached its obligations to secure from counsel a tax opinion, which would have allowed the companies’ planned $37.5 billion merger to proceed.

Sandra C. Goldstein, who argued Williams’ appeal in Dover, said that provisions in a September 2015 merger agreement required Energy Transfer to take “all acts necessary” to compel its Latham & Watkins tax attorneys to issue an opinion that critical elements of the transaction should be treated as tax-free.