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Diborane B₂H₆ has only 12 valence electrons but 8 atoms to connect. How? Step 1: Normal bonds need 2 e⁻ each; 7 bonds would need 14 e⁻ — we have only 12! Step 2: Four terminal B–H bonds use 8 e⁻ (normal 2c–2e). Step 3: Remaining 4 e⁻ form TWO three-centre B–H–B bridges (2 e⁻ each). Step 4: These 'banana bonds' are bent — electron density curves like a banana. Electron-deficient solved!