Q.In diborane, the number of electrons that accounts for banana bonds is

(a) Diborane's structure has 4 normal B-H bonds plus 2 electron-deficient 3-centre-2-electron bridge bonds, and its ethylborate test gives a green flame. (b) Metal-carbonyl bonding is synergic: sigma donation from CO to the metal plus pi back-donation from the metal to CO's antibonding orbital.

(a)(i) Structure of diborane, $B_2H_6$: Each boron atom is $sp^3$ hybridised. Diborane has 4 terminal hydrogens, two on each boron, bonded by normal 2-centre-2-electron (2c-2e) covalent B-H bonds lying in a plane. The remaining 2 hydrogens bridge the two boron atoms above and below this plane; because boron is electron-deficient (each B contributes only 1 electron and one orbital to each bridge), these two B-H-B bridge bonds are unusual 3-centre-2-electron (3c-2e) 'banana' bonds, where a single pair of electrons is delocalised over three atoms (B-H-B) rather than localised in one B-H bond. This gives diborane its overall structure: a central $B_2H_2$ bridging unit flanked by two terminal $BH_2$ groups.

(a)(ii) Ethylborate test: A borate/boric-acid sample is heated with ethanol ($C_2H_5OH$) in the presence of concentrated sulphuric acid (which acts as a dehydrating/catalysing agent). This forms volatile ethyl borate, $B(OC_2H_5)_3$, whose vapour, when ignited, burns with a characteristic green-edged flame — a standard qualitative test confirming the presence of borate.

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