Dative Covalent Bonding in BCl3?

Dative covalent bonding is when both electrons in the covalent bond come from the same atom.

As boron has 3 electrons in its outer shell it can form 3 covalent bonds with each chlorine where each atom donates 1 electron to the bond so each chlorine can gain a full outer shell.

Recently I answered an exam style question which asked me to draw a dot and cross diagram for BCl3, showing outer electrons only.

The actual answer showed a boron atom in the centre, with 3 covalent bond electron pairs surrounding it from each chlorine atom.

Following the octet rule, I drew an extra two electrons from one of the chlorine atoms to signify a dative covalent bond, however this seems to be incorrect.

Why is it incorrect, and in what situation would I use dative covalent bonding?

James

I understand that each chlorine atom has a full shell through covalent bonding, but by boron forming 3 covalent bonds it would only have 6 electrons in its outer shell (3 from its original outer shell and 1 from each chlorine atom, so an extra 3).

But why doesn't one of the chlorine atoms share 2 of its own electrons via a dative covalent bond so that boron has a full outer shell (since 6+2=8)?

Good question (and studiously ignored by the subsequent posters).

The electron deficient structure only occurs in the gaseous state, while the solid boron trichloride has tetrahedral boron atoms with full octets in a polymeric structure.

Why can it not form a dative coordinate bond with a chlorine lone pair?

The answer is because it is energetically unfavourable to do so ... but why is that the case?

I can only imagine that is has something to do with the electropositive nature of the boron atom (electronegativity 2.04) not being attractive enough for the electron pair compared to the electronegative chlorine atom (electronegativity 3.16) .

Boron also has no available 'd' orbitals to make the process easier. It does have an available 'p' orbital but, that is apparently not good enough in the gaseous state.