PURPOSE: To well obtain the subject compd. in an industrial scale by bringing a hexaaryldiphosphonium compd. into contact with an aluminum alkali metal tetrahydride or a compd. derived therefrom in the presence of an aprotic diluent.

CONSTITUTION: One mol. of a hexaaryldiphosphonium compd. represented by formula I (wherein R is a divalent crosslinking group containing two or more C-atoms in its crosslinking, R1 is an aryl having an alkoxy substituent present at an ortho position with respect to P to which the aryl is bonded and X is a halogen) and 0.5 mol. of an aluminum alkali metal tetrahydride or a compd. derived therefrom by substituting 1-3 H-atoms of the aluminum alkali metal tetrahydride with OR2 (R2 is an alkyl or an alkoxyalkyl) are reacted pref. in the presence of an aprotic diluent (e.g. THF, benzene) at high temp. under reflux to obtain an objective compd. represented by formula II being one kind of a component in a catalyst compsn. useful in the production of a polymer composed of an olefinic unsaturated compd. and carbon monoxide.