▁re agent . ▁ ▁The ▁di an ion ▁is ▁is oe lect ron ic ▁with ▁Ni ( CO ) 4 ▁but ▁highly ▁nucle oph il ic . ▁ ▁Ac id - base ▁re actions ▁Fe ( CO ) 5 ▁is ▁not ▁readily ▁pro ton ated , ▁but ▁it ▁is ▁attacked ▁by ▁hydro x ide . ▁ ▁Tre at ment ▁of ▁Fe ( CO ) 5 ▁with ▁a que ous ▁base ▁produces ▁[ H Fe ( CO ) 4 ] − , ▁via ▁the ▁Met alla car box yl ate ▁intermediate . ▁The ▁ox id ation ▁of ▁this ▁mon o an ion ▁gives ▁tri ir on ▁d ode car b ony l , ▁Fe 3 ( CO ) 1 2 . ▁ ▁Ac id ification ▁of ▁solutions ▁of ▁ ▁[ H Fe ( CO ) 4 ] − ▁gives ▁iron ▁t et rac ar b ony l ▁hyd ride , ▁H 2 Fe ( CO ) 4 , ▁the ▁first ▁metal ▁hyd ride ▁ever ▁reported . ▁ ▁D iene ▁ad duct s ▁D ien es ▁react ▁with ▁Fe ( CO ) 5 ▁to ▁give ▁( d iene ) Fe ( CO ) 3 , ▁where in ▁two ▁CO ▁lig ands ▁have ▁been ▁replaced ▁by ▁two ▁o lef ins . ▁ ▁Many ▁di enes ▁under go ▁this ▁reaction , ▁not ably ▁nor born ad iene ▁and ▁ 1 , 3 - but ad iene . ▁ ▁One ▁of ▁the ▁more ▁histor ically ▁significant ▁derivatives ▁is ▁cy c lob ut ad iene ir on ▁tr ic