ochem question

[glme]

An O-Ga bond formation between ethyl methyl ether and GaI3 is a coordinate covalent bond.


Why is that so? Ga is a metal, so shouldn't a bond between a nonmetal and a metal be an ionic bond?


(thanks so much for your help in advance)

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[FeralisExtremum]

While it's true that metal and nonmetal atoms generally form ionic bonds, it is also possible for coordinate covalent bonds to be formed between metal and nonmetal atoms (usually when at least one of the atoms which are bonding are part of molecules). In coordinate covalent bonding one molecule usually has an atom with a lone pair of electrons (the electron pair donor or Lewis base) and the other atom is a metal ion that acts as the electron pair acceptor (Lewis acid). They key difference being that one atom is providing both electrons that are shared in the coordinate covalent bond, whereas in an ionic bond there's a complete transfer of electrons as opposed to sharing,

 

[DAT Destroyer]

An O-Ga bond formation between ethyl methyl ether and GaI3 is a coordinate covalent bond.


Why is that so? Ga is a metal, so shouldn't a bond between a nonmetal and a metal be an ionic bond?


(thanks so much for your help in advance)

You have asked a wonderful question. Gallium is an unusual metal. It has a melting point of about 30 C.....and will melt into a silverly liquid if placed in your hands on a warm day !! Gallium has an electronegativity of 1.6. It does indeed form ionic compounds with other atoms such as Oxygen. Gallium oxide has an electronegativity difference about 2...clearly over the 1.7 value usually given for ionic compounds. When reacted with nucleophiles such as chlorine, bromine, or iodine,,,,,,the VACANT 4p orbital is attack. The nucleophile gives BOTH electrons to this vacant orbital, hence we form the coordinate covalent bond. As a nice rule of thumb that I teach my students is that elements of Group 13, especially B, Al, Ga, and In have orbitals that are frequently attacked by nucleophiles and form coordinate covalent bonds. In Advanced Organic chemistry, we sometimes need to choose various Lewis acids from this group.

Hope this helps.

Dr. Romano