Microsoft is set to challenge Google in the online ad world with the sealing of its US$6 billion takeover of Internet advertising firm aQuantive.

The software maker closed the deal of its largest purchase in history on Friday, according to a filing aQuantive made on Monday in the US with the Securities and Exchange Commission (SEC).

In the filing, Seattle-based aQuantive said that its board has resigned, as planned, and that it has notified Nasdaq to delist its stock, with the shares now converted into the right to receive a cash payout of US$66.50 per share from Microsoft. aQuantive is now a wholly owned subsidiary of Microsoft.

Microsoft is using the close of the aQuantive deal to shuffle around some of its leadership positions and duties within the online-advertising business. Yusuf Mehdi, who had been the company's chief advertising strategist, will shift to a new role, heading strategic partnerships, according to an internal memo sent on Monday morning in the US by Platforms & Services Division president Kevin Johnson.

Steve Berkowitz, the former Ask.com chief executive, continues to lead Microsoft's Online Services Group, though responsibility for ad platform product management, search and display marketing, and mobile and gaming advertising are moving to Karl Siebrecht, an aQuantive unit head.

Meanwhile, aQuantive chief executive Brian McAndrews will lead a newly formed Advertiser and Publisher Solutions group. Siebrecht will report to McAndrews, while McAndrews, Berkowitz and Mehdi will all report to Johnson.

With the formalities out of the way, Microsoft will now look to try to get a boost for its ad-based efforts as it absorbs aQuantive, which is basically made up of three businesses:

• Avenue A Razorfish is an internet advertising agency.
• Atlas Media Console helps advertisers and ad agencies buy and manage online ads.
• Drive Performance Media buys large blocks of advertising and then resells them to advertisers based on specific criteria.

"Today we take a significant step forward in our ability to capture share of the US$40 billion online-ad opportunity and the larger US$600 billion ad market, which is rapidly shifting to the world of online and IP-served platforms, including TV and gaming," Johnson said in a statement. "The addition of aQuantive's technologies and people to the Microsoft portfolio is a core, strategic investment, and step forward in our plans to become one of the top two online-advertising platforms in the industry."

Microsoft's decision to buy aQuantive earlier this year followed a flurry of deals in the online ad business, including Google's plan to purchase DoubleClick for US$3.1 billion -- a deal that still awaits regulatory approval. Last month, AOL said it is buying Tacoda, while Yahoo said it will spend US$680 million to buy the 80 percent of Right Media that it didn't already own.