The following obligations cannot be specifically enforced: (1) An obligation to render personal service; (2) An obligation to employ another in personal service; (3) An agreement to submit a controversy to arbitration; (4) An agreement to perform an act which the party has not power lawfully to perform when required to do so; (5) An agreement to procure the act or consent of the wife of the contracting party or of any other third persons; or (6) An agreement, the terms of which are not sufficiently certain to make the precise act which is to be done clearly ascertainable. (Code 1923, §6833; Code 1940, T. 9, §55.)