'unprecedented chance' is correct and usable in written English.
You can use it when you mean that someone has an opportunity that is extraordinary or unique. For example, "This is an unprecedented chance for a career breakthrough in the industry."
It also gives the Liberal Democrats an unprecedented chance to reach a huge audience.
To writers such as Murland, the centenary is an unprecedented chance to communicate a passion.
Nonetheless, they say, taken together the package represents "an unprecedented chance to put all this right".
Southern Sudan has suffered a lot, and after years of civil war, oppression and displacement, many people here saw the vote as an unprecedented chance at self-determination.
A joint venture by the festival and, from Wagner's birthplace, Opera Leipzig, the project offered an unprecedented chance to experience these rarely performed works in close proximity to each other.
In an open letter to BNP supporters, he said his appearance on the flagship BBC1 political show was an unprecedented chance to present their views to the UK public.
It also presents an unprecedented opportunity -- the chance to build a prosperous, clean energy economy.
The World Meteorological Organization WMOO) says "this is all bad news for the planet" but I believe now we have in front of us is an unprecedented opportunity, and historical chance to make the paradigm shift that humanity desperately needs.
In an extraordinary move he asked the firms how best to draft rebuttals to criticisms of government proposals that might arise from its consultation on legal aid reforms, effectively offering them an unprecedented second chance to shape policy.
The world today has an unprecedented opportunity to reduce child mortality in the poorest countries – a chance that cannot be missed.
There were hundreds of thousands of engineering and science graduates in the country, and the software industry sucked them in at various rungs, offering them unprecedented salaries and a chance to work in the United States.
