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 except as authorized in the Uniform Arbitration Act;
(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 spouse of the contracting party, or of any other third person;
(6) An agreement, the terms of which are not sufficiently certain, to make the precise act which is to be done clearly ascertainable.