Elements diagonally placed show similar properties due to comparable charge/size ratios. B-Si: Both covalent, form acidic oxides (B2O3, SiO2), chlorides hydrolysed, form complex hydrides (NaBH4, NaAlH4 — wait, B-Si analogy: both form volatile hydrides and polymeric oxides). Li-Mg: Similar ionic radii, both form nitrides on heating in N2, both carbonates decompose on heating, LiOH is weak base like Mg(OH)2. Be-Al: Both form amphoteric oxides, both form covalent chlorides (BeCl2 polymeric, AlCl3 dimeric), both form [M(OH)4]^- in excess NaOH, both show max covalence of 4.