The following obligations cannot be specifically enforced:
(a) An obligation to render personal service.
(b) An obligation to employ another in personal service.
(c) An agreement to perform an act which the party has not power lawfully to perform when required to do so.
(d) An agreement to procure the act or consent of the spouse of the contracting party, or of any other third person.
(e) An agreement, the terms of which are not sufficiently certain to make the precise act which is to be done clearly ascertainable.