Selective fluorination of o0 and m-carboranes. Synthesis of 9-monofluoro-, 9,12-difluoro-1,8,9,12-trifluoro-, and 8,9,10,12-tetrafluoro-o-carboranes and 9-monofluoro-, and 9,10-difluoro-m-carboranes. Molecular structure of 8,9,10,12-tetrafluoro-o-carborane