Legally separated is not a filing status. You either file married or single (possibly head of household if you have qualifying dependents). The IRS considers you unmarried if you have a legal separation agreement on the last day of the year. IRS Pub 501 has complete information about filing status to include information if you think you qualify as head of household.
https://www.irs.gov/publications/p501/ar02.html#en_US_2016_publink1000220721
Marital Status
In general, your filing status depends on whether you are considered unmarried or married.
Unmarried persons.
You are considered unmarried for the
whole year if, on the last day of your tax year, you are either:
State law governs whether you are
married or legally separated under a divorce or separate maintenance
decree.
