First Niagara Financial Group will not replace a departing top executive who specialized in helping the bank find and complete acquisitions, another indication the bank is focused on digesting its recent acquisitions rather than pursuing more.

The Buffalo-based bank announced in a regulatory filing with the Securities and Exchange Commission that Oliver H. Sommer, its executive vice president of corporate development, will be leaving the company June 30. Other executives at the bank will absorb oversight of his team and related functions, the bank said.

Sommer had been working with the bank for eight years, first as an outside adviser to President and CEO John R. Koelmel through Sommer’s firm, Aston Associates, and then as an executive for the last three years, since April 2010.

During his time with First Niagara, the bank more than quadrupled in size to its current $37 billion in assets, as it completed two major acquisitions in western and then eastern Pennsylvania, followed by its purchase of NewAlliance Bancshares in New Haven, Conn., and finally its purchase of HSBC Bank USA’s entire upstate New York branch network, with 195 branches.

After the acquisitions, in an effort to reassure Wall Street, executives have been insisting that they are done expanding for a while and are focused on basic banking.

“Mr. Sommer was instrumental in leading our acquisition strategy over the past several years and has significantly contributed to our success in his corporate development leadership role,” spokesman David Lanzillo said.“As we have said, we are very much focused on effectively running the company we have today.”

Last year, Sommer earned a salary of $512,500, up from $488,462 the previous year. His full compensation for 2011, including stock, was $1.34 million, but the full compensation for 2012 isn’t yet available.