The reaction of the phosphinidene complex [Cp∗P{W(CO)₅}₂] (1 a) with di-tert-butylcarboimidophosphene leads to the P -C cage compound 6 and the Lewis acid-base adduct [Cp∗P{W(CO)₅}₂(CNtBu)] (2 a). In contrast, the arsinidene complex shows a different reactivity. At low temperatures, the arsaphosphene complex [{W(CO)₅}{η²-(Cp∗)As =P(tBu)}{W(CO)₅}] (3) is formed. At these temperatures, 3 reacts further with a second equivalent of carboimidophosphene to form [{W(CO)₅}{η²-{(Cp∗)(tBu)P}As =P(tBu)}{W(CO)₅}] (5), probably by the insertion of a phosphinidene unit (tBuP) into an As -C bond. In contrast, at room temperature 3 reacts further by a radical-type reaction to form [{(tBu)P =As -As =P(tBu)}{W(CO)₅}₄] (4). Compound 4 is the first example of a neutral, 1,3-butadiene analogue containing only mixed heavier Group 15 elements. It consists of two P=As double bonds connected by arsenic atoms.