3c-2e bonds: In B2H6, two bridge H atoms each form a banana bond — 2 electrons shared among B-H-B (3 centres). This is unique to electron-deficient boron compounds. Backbonding in BF3: F lone pairs donate into empty p-orbital of B (ppi-ppi), making B-F bond partially double (B-F shorter than expected). BF3 is therefore less Lewis acidic than BCl3 (Cl is larger, poorer overlap for backbonding). PCl5 structure: Trigonal bipyramidal in gas, but [PCl4]^+[PCl6]^- in solid (ionic). N2O5: Ionic in solid as [NO2]^+[NO3]^-. P4 structure: Tetrahedral with 60° P-P-P angles (highly strained, hence reactive). P4O10: Cage structure derived from P4 with O bridges and terminal P=O bonds.