▁Lewis ▁acid ▁and ▁thus ▁in active ▁as ▁a ▁catal yst ▁for ▁Fried el - C raft s ▁re actions . ▁ ▁Sam ples ▁of ▁Al Cl 3 ▁must ▁therefore ▁be ▁protected ▁from ▁atmos pher ic ▁mo ist ure ▁to ▁pre clude ▁the ▁formation ▁of ▁hyd r ates . ▁ ▁Cry st als ▁of ▁hyd r ated ▁co pper ( II ) ▁sul f ate ▁consist ▁of ▁[ C u ( H 2 O ) 4 ] 2 + ▁cent ers ▁linked ▁to ▁SO 4 2 − ▁ ions . ▁ ▁Co pper ▁is ▁surrounded ▁by ▁six ▁o xygen ▁atoms , ▁provided ▁by ▁two ▁different ▁sul f ate ▁groups ▁and ▁four ▁mole cules ▁of ▁water . ▁ ▁A ▁fifth ▁water ▁res ides ▁elsewhere ▁in ▁the ▁framework ▁but ▁does ▁not ▁bind ▁directly ▁to ▁co pper . ▁ ▁The ▁c ob alt ▁ch lor ide ▁mentioned ▁above ▁occurs ▁as ▁[ Co ( H 2 O ) 6 ] 2 + ▁and ▁Cl − . ▁ ▁In ▁tin ▁ch lor ide , ▁each ▁Sn ( II ) ▁center ▁is ▁py ram idal ▁( mean ▁O / Cl - S n - O / Cl ▁angle ▁is ▁ 8 3 ° ) ▁being ▁bound ▁to ▁two ▁ch lor ide ▁ ions ▁and ▁one ▁water . ▁ ▁The ▁second ▁water ▁in ▁the ▁formula ▁unit ▁is ▁hydro gen - b ond ed ▁to ▁the ▁ch lor ide ▁and ▁to ▁the ▁coordin ated ▁water ▁mole c ule . ▁ ▁Water ▁of ▁cry st all ization ▁is ▁stabil ized ▁by ▁electro static