An inorganic compound 'A' shows the following reactionsi) It is white solid, exists as dimer and fumes in wet airii) It sublimes at 1800C and forms monomer if heated to 4000Ciii) Its aqueous solution turns blue litmus to red and gives a white precipitate with AgNO3 solution which is soluble in NH4OHiv) Addition of NH4OH and NaOH separately to the soltion of 'A' gives a gelatinous precipitate which in however soluble in excess of NaOH.The compound 'A' is