[" In which of the following pairs the iwo species are not isostructural? "],[[" (1) "CO_(3)^(4)" and "NO_(3)," (2) "PCl_(4)^(prime)" and "SiCl_(4)],[" (3) "PF_(6)" and "BrF_(8)," (4) "AlF_(6)^(3)" and "BF_(6)]]