MadSci Network: Chemistry

Re: Is there a case where >4 bonds are made to a single atom?

Date: Tue Feb 24 01:04:19 1998
Posted By: Jeremy Starr, Grad Student, Chemistry, California Institute of Technology
Area of science: Chemistry
ID: 886054601.Ch

Hi Daniel,


There are many molecules containing atoms that have more than four bonds to
them. The characteristic that makes it possible for a particular element to
form more than four bonds is the presence of d-orbitals. Since all elements
after Neon, that is all elements of atomic number 11 or higher, possess (in
theory) either an empty, or partially filled, set of d-orbitals, the _only_
elements restricted to four bonds or less per atom are Hydrogen through
Neon (the first two rows of the periodic table). One notable exception to
this is boron which readily forms "clusters" containg 3-center 2-electron
bonds allowing each boron to have more than four equivilent bonds to other
atoms; however, these bonds are not conventional two electron covalent

Bonding orbitals are formed by linear combination of atomic orbitals in the
valence shell. Not all orbitals in the valence shell must combine but, as a
rule, the number of bonding orbitals formed exactly equals the number of
atomic orbitals combined. Hydrogen and Helium have only one valence orbital
and thus can form a maximum of one bond to another atom. Lithium through
Neon have one S orbital and 3 P orbitals totalling four atomic orbitals in
their valence shells. Thus a maximum of four bonding orbitals can be
formed, limiting the number of bonds to each of those elements to four.
Other factors contribute to further limitations of the number of bonds
certain elements in that row can form but in no case will any element in
that row form more than four conventional two electron covalent bonds. 

Elements in later rows have 5 D orbitals in addition to 1 S orbital and 3 P
orbitals in their valence shells. Numbers of linear combinations of these
orbitals can easily exceed four and frequently do. The fields of inorganic
and organometallic chemistry depend on the ability of transition metals
(center section of periodic table) to form many more than four coordinate
covalent bonds. Inorganic non-metals (lower right of periodic table) also
form compounds with more than four bonds to an atom. Hexafluorosilicate
(SiF6)2- and PCl5 are both examples of these. If I wasn't limited by the
text format of the answer box I would draw the structure of the Dess-Martin
periodane, a common reagent in organic synthesis which has five bonds to

There are many, many, many stable compounds that have more than four bonds
to a single atom. Simply flip through any organometallic chemistry textbook
to find an abundance of them.

I hope this information was helpful.


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