Russian companies have made $180 billion in deals globally in the past two years, providing steady profits to London bankers, lawyers, and image crafters as the city has become a hub for such transactions. Sanctions planned by the U.S. and European Union threaten that business.

The potential fallout highlights the web of connections linking Russia to the global financial system. Since many large Russian companies are controlled by the state or by billionaires with close ties to President Vladimir Putin, even narrowly targeted sanctions could hurt their global operations.

A reminder of the stakes emerged on March 16, when L1 Energy, a London-based investment vehicle backed by Russian billionaire Mikhail Fridman, agreed to buy Dea, the oil and gas unit of Germany's RWE AG, for $7.1 billion—the biggest Russia-related deal this year.