[Rayan]

why doesnt carbon atom form any complexes?

[woelen]

why doesnt carbon atom form any complexes?

Wouldn't you regard the carbonate ion as a complex? You can regard it as a complex of three oxo-ligands, coordinated strongly to one carbon-core  

OK, just kidding... carbonate can form complexes, but not as a simple ion, simply because there is no simple ion of carbon. The carbide ion C^4- is quite rare. There are, however, many many complexes with ligands, which contain carbon. Some to mention:
- organometallic complexes, most notably the carbonyl complex M(CO)_n, with M being some metal.
- oxalato complexes, such as the nice green trisoxalatoferrate (III) ion.
- carbonato complexes of cobalt (III).
- acetylido complexes, such as Cu₂C₂.

Now coming back to the carbonate ion itself. The border between complex and standard bond is not a sharp border. Usually, the ions like sulfate, carbonate, sulfite, nitrate, etc are regarded as ions, containing covalent bonds and hence, these ions can be regarded as molecules with some net charge. A nice non-oxo example is the ion Fe(CN)₆^3-. This usually is regarded a complex, but the CN-groups are connected to the iron so tightly that one equally well could speak of a normal bond, such as the one encounted in sulfate ion. Another example is TiF₆^2-. The element titanium in its +4 oxidation state acts towards the fluoride ligand in a similar way as e.g. sulphur acts towards the oxo-ligand.
The only reason why we think that oxo-anions are not complexes, is that they are so common, but the hexafluorotitanate ion is of similar stability as e.g. nitrate or nitrite and one can make salts of this, such as K₂TiF₆. Many people do not regard the titanate ion, TiO₃^2-, a complex, while they do regard the fluoro-titanate ion as a complex.

A really nice example of a complex is the mixed fluoro-oxo ligand complex, called fluoro-phosphate. This is the ion PO₃F^2-. This is a very stable ion and salts of this, such as Na₂PO₃F can be purchased commercially (in fact, it is used in some toothpastes). In the fluoro-phosphate ion, three oxo-ligands and one fluoro-ligand are arranged around a single phosphor core in oxidation state +5 (the most stable oxidation state for phosphorus).
Now, would you call this a complex or a standard ion, such as phosphate. If you call this a complex, why not call plain phosphate a complex?

[Mitch]

why doesnt carbon atom form any complexes?

Metal carbides are a common type of material, although the term complex usually refers to non-metalic bonding.

[mip]

The border between complex and standard bond is not a sharp border.

Ions or compounds are "complex" when the coordination number (i.e. number of ligands) is greater than the oxidation number the metal ion would have if the ligands were removed.

[woelen]

Ions or compounds are "complex" when the coordination number (i.e. number of ligands) is greater than the oxidation number the metal ion would have if the ligands were removed.

This is an interesting definition of the concept of "complex". I've not seen this before, but I tried for several compounds and it works out nicely.

There are a few border-line compounds though, where I still have questions. Two examples:

PO₃F^2- often is called a complex. According to your definition it is not, because #ligands=4, ox.nr=5.

CrO(O2)2 also frequently is called a peroxo-complex of chromium. There is one mono-dentate oxo-ligand and two bidentate peroxo-ligands, so the coordination number is 5, while the ox.nr of chromium equals 6. According to your definition it is not a complex.

I tend to agree with you and say that these are not complexes. Could you shed some more light on how these are handled?

According to your definition, compounds like diborane (B2H6, better notation: H3BBH3) and NI3.xNH3 would be complexes? Also an ion like NH4(+) would be a complex (#ligands=4, ox.nr of H equals -3, sign of ox.numbers reversed for central atom and ligands, but principle remains same).

[mip]

Hmm, I'm not 100% sure.  That definition was the one my lecturer gave me when we were studying coordination compounds.

I might be wrong, but I was under the impression that coordination chemisty was only relevant for transition metals, which would answer your question about diborane etc. as boron and nitrogen are main group elements.

[woelen]

Hmm, I'm not 100% sure.  That definition was the one my lecturer gave me when we were studying coordination compounds.

I might be wrong, but I was under the impression that coordination chemisty was only relevant for transition metals, which would answer your question about diborane etc. as boron and nitrogen are main group elements.

No, I see no reason, why complexes are restricted to transition metals. Any element can form complexes (e.g. SnCl4(2-), BiI4(-), but also the elements, higher up in the periodic table, with complexes like [Ca-EDTA](2-) and [Mg-EDTA](2-).

[mip]

Ok, so as far as I can tell we're both kinda right  

I found this definition at http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch12/complex.html

"Coordination compounds, such as the FeCl^4- ion are called such because they contain ions or molecules linked, or coordinated, to a transition metal. They are also known as complex ions or coordination complexes because they are Lewis acid-base complexes. The ions or molecules that bind to transition-metal ions to form these complexes are called ligands (from Latin, "to tie or bind"). The number of ligands bound to the transition metal ion is called the coordination number.

Although coordination complexes are particularly important in the chemistry of the transition metals, some main group elements also form complexes. Aluminum, tin, and lead, for example, form complexes such as the AlF₆^3-, SnCl₄^2- and PbI₄^2- ions."

[mip]

FeCl^4-

Oops... this is obviously supposed to be FeCl₄^2-

[woelen]

Oops... this is obviously supposed to be FeCl₄^2-

No, the deep yellow FeCl₄^- probably is meant here . This contains iron in its +3 oxidation state. You can see its formation very nicely, by dissolving some ferric sulfate or ferric ammonium sulfate in very dilute sulphuric acid and then adding a few drops of dilute HCl.