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.