What constitutes a Legal seperation?

No. "legal" separation would probably require you to go to court and obtain a separate maintenance (or divorce) decree. then you could file as single if the decree was in effect before the end of the year.

there is separation (not "legal")  for income tax purposes but that requires living part for at least the last 6 months of a tax year and having paid more than 1/2 the cost of keeping up a household for themselves and their child, who can be claimed as a dependent, and 546=+who lived with them for more than 6 months of the year. then you can file as head of household. if not your filing status can be either joint if both agree or married filing separately.