There are no real "grounds" for impeachment, because it is not a criminal charge. It is a purely political act and, literally, anything can be cited as the cause of action...the charge could be "failing to trim his toenails" for example. In legal terms, it is VERY true that the president cannot commit the crime of obstruction nor collusion. Those terms can be tossed into an impeachment article, but so could "swiss cheese" or "bananas". It is not a legal case that is made, but rather, a political one.