|MadSci Network: Chemistry|
Hi Daniel, Yes. 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 bonds. 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 iodine. 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. Jeremy.
Try the links in the MadSci Library for more information on Chemistry.