The corresponding reactions are 4𝐵𝐹3+3𝐿𝑖𝐴𝑙𝐻4→2𝐵2𝐻6+3𝐿𝑖𝐹+3𝐴𝑙𝐹3 3𝐵2𝐻6+6𝑁𝐻3→3[𝐵𝐻2(𝑁𝐻3)2]+[𝐵𝐻4]− 3[𝐵𝐻2(𝑁𝐻3)2]+[𝐵𝐻4]−+ℎ𝑒𝑎𝑡→2𝐵3𝑁3𝐻6+12𝐻2 So here the Lewis acid `X’ is 𝐵𝐹3 , inorganic benzene is 𝐵3𝑁3𝐻6 which is also know as Borazine. So the toxic gas is 𝐵2𝐻6