Kansas City, Mo. ? Sprint Nextel Corp. on Tuesday announced that Tim Donahue is leaving as chairman of the struggling cell phone carrier, the second unexpected departure of a top executive in six weeks.

The company did not name a replacement for Donahue, 57, who was chief executive officer of Nextel before it was acquired by Sprint last year for $35 billion, creating the nation’s third-largest cellular provider with more than 40 million wireless customers and $40 billion in annual revenue.

Donahue’s resignation, effective Dec. 31, surprised some analysts, but a Sprint spokesman said there had been speculation since the two companies merged that Donahue would stay for only two years. The spokesman stressed that the resignation was voluntary and had been expected internally.

The announcement came on the heels of a recent rough patch, including weak second-quarter earnings and the ouster of Len Lauer, a veteran Sprint executive, as president. Sprint Nextel CEO Gary Forsee and Donahue orchestrated the merger of the companies.

Sprint’s shares, down about a third from their 52-week peak, slipped 4 cents to $18.04 in Tuesday’s trading on the New York Stock Exchange.

The departure of Donahue, a Nextel executive since 1996, “will make the integration of Sprint and Nextel that much more challenging,” Greg Miller, a telecom analyst with Deutsche Bank, said in a report.