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    A dative covalent bond occurs between BCl3 and PH3-. Explain how this occurs.
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    Take a look at the outer shell of boron atoms, you'll notice that boron only has 3 valence electrons. This means that when it's bonded all three of them it is still 2 electrons short of a full octet. Phosphorus on the other hand can form 3 normal covalent bonds and has a lone pair so it achieves a full octet. If the phosphorus atom in it's moleculehappens to bump into an electron deficient boron atom in another molecule in suitable conditions, it forms a dative covalent bond between the boron and phosphorus atoms. The lone pair on the phosphorus is shared between the two atoms so that both have a full octet. This makes the boron's electron arrangement much more stable.

    Here's a dot and cross diagram of boron http://www.benjamin-mills.com/teachi...n-electron.png

    And one of phosphorus http://f.tqn.com/y/chemistry/1/S/D/9...phorusatom.jpg
 
 
 
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