Upon what facts do you base this assertion?
I do not recall any fact based charges of Nixon being a crook.
The charges leveled against him had to do with the Watergate cover up and various abuses of power but nothing about him being a crook.
It was an unfounded aspersion tossed around by his enemies, much like the canard about Sarah Palin saying she can see Russia from her house.
You can read the articles of impeachment here:
Nixon had his people break into the Watergate to steal documents from Democrats. That walks and quacks like a crook...