Monsanto just added billions to an offer to buy a Swiss rival and it still won't budge

Monsanto is trying again.

The giant agribusiness best known for its pesticides and genetically modified crop seeds is once again pursuing a merger with Syngenta, one of the world's largest manufacturers of agricultural chemicals.

The latest offering from Monsanto is a cash and stock deal worth about $447 (449 Swiss Francs) per share. That's a more than 45% premium over Syngenta's 52-week average share price.

Monsanto appears to be eager to win over Syngenta's skeptical advisers.

A person close to Syngenta told Reuters that Syngenta's board members are worried about the regulatory hurdles a potential merger would bring, considering the combined companies would control nearly half the seeds market in the US.

To that end, Monsanto has added a $2 billion reverse breakup fee that it would pay Syngenta if the deal fails the antitrust sniff test.

Advisers for both Monsanto and Syngenta have reportedly met in New York as recently as one week ago but, to date, no substantial headway has been made.

St. Louis-based Monsanto has been trying to reel in Syngenta since 2014, touting the potential for both companies to "create significant value for growers to ultimately meet the needs of broader society," Monsanto CEO Hugh Grant said in an emailed statement.

But, among the obstacles that could derail the merger are concerns by US lawmakers that it might open the door for Monsanto to move its headquarters out of the US; a strategy sometimes used by large companies seeking lower corporate taxes.

Interestingly, another wrinkle underlying Syngenta's reluctance to dance with Monsanto is reputation. Reuters reported Friday that the "regulatory scrutiny and consumer backlash" Monsanto already faces globally is enough to make Syngenta's executives pause, regardless of how many billions of dollars are added to the deal.