What is structural feature of monomeric and polymeric Beryllium halides ?

All halides  of 2nd group elements are ionic in nature except Beryllium halides. BeCl₂ is soluble in organic solvents and anhydrous BeCl₂ has polymeric structure, in vapour phase exist as monomer and also as dimer. 

Bond length data show that dimer and  polymer has a  three center two electron chloro (Be-Cl-Be)bridge bond .  which at high temperatures of the order of 1200 K dissociates into linear monomer. 

The tendency to form halide hydrates gradually decreases down the group.

For example:

MgCl₂.6H₂O

CaCl₂.6H₂O

SrCI₂.6H₂O

BaCl₂.2H₂O 

(1)The hydrated chlorides, bromides and iodides of Ca, Sr and Ba can be dehydrated by heating but the corresponding hydrated halides of Be and Mg on heating undergoes hydrolysis.

(2) The fluorides of alkaline earth metals are less soluble than chlorides due to their high lattice energies.