When there is no eligible list from which a position may be filled, the appointing power may, with the consent of the commission, fill such position by temporary appointment. A temporary appointment shall not continue for a longer period than three months, nor shall successive temporary appointments be made to the same position under this section without the previous consent of the commission. In no case shall any person hold a position under a temporary appointment for a period longer than six months without the unanimous consent of the commission. All temporary appointments caused by leaves of absence shall be made from the eligible list of the classified service.