Sigg's collection will be a cornerstone of the museum's collection. An agreement further states that around 500 works from the collection will be displayed every year in the 5,000-square-meter exhibition hall.
Sigg's gift is conservatively valued at around US$163 million; the museum's additional purchase is valued at US$22.9 million.
Last year Baron Guy and his wife Myriam Ullens sold 106 works of contemporary Chinese art at Sotheby's for US$46.6 million; they plan to auction the rest over time.
The Sigg collection contains the works of around 350 artists, including Ding Yi, Fang Lijun, Geng Jianyi, Gu Wenda, Huang Yongping, Liu Wei, Wang Guangyi, Xu Bing, Yang Shaobin, Yue Minjun, Yu Youhan, Zeng Fanzhi, Zhang Peili and Zhang Xiaogang.
Sigg told an earlier press conference he had chosen Hong Kong "for its freedom and its proximity to mainland audiences."
"I'd had a number of exchanges with mainland institutions," Sigg was quoted as saying by The New York Times on June 12. "My first impulse was to think of the mainland. But the conditions are not such that art could be shown without limitations."
He said he preferred to donate to a brand-new Hong Kong project instead of an established Western museum.
"It's very important that a Chinese public have access to these works," he said. "The Chinese public ought to see its own contemporary art, which it doesn't know yet."
Sigg said in a statement, "By joining forces with M+, the art works will ultimately come full circle back to China as I have always hoped they would. My intention is to return something to China for what it has allowed me to experience over the last 33 years: an incredible journey."
Questions sparked
The donation and cooperation between museum and art collector have generated controversy, though Chinese art and culture officials and mainland museums have not commented. There's heated online debate by individuals, many of whom resent that the collection is going to Hong Kong and not staying on the mainland.
"This kind of donation is cheating. Actually Sigg is turning a public art museum into his private museum and the Kong Kong government is wasting taxpayers' money," wrote Jiang Yinfeng, an art critic based in Guangzhou in an article in the Oriental Morning Post newspaper on July 16.
In the view of many Chinese, donation by its nature should be generous and selfless, without strings attached, such as additional sale and the agreement on exhibiting 500 works a year.