 It is a moderately strong Lewis acid , forming adducts with Lewis bases such as triphenylphosphine oxide , e.g. FeCl3 ( <unk> ) 2 where Ph = phenyl . It also reacts with other chloride salts to give the yellow tetrahedral <unk> − ion . Salts of <unk> − in hydrochloric acid can be extracted into diethyl ether . 
