Philip Power offers a new insight into the chemistry of some of the heavier main group elements - that square in the periodic table cornered by aluminium and tellurium - in a recent review in Nature (DOI). In it he proposes that based on the type of bonding and reactivity compounds containing elements in this square can really behave as transition metals.
Power starts his investigation with the chemistry of some compounds containing two such heavier main group elements joined by a multiple bond such as mes2Si=Simes2. On going down a group (Si > Ge > Sn > Pb) the M-M multiple bond bond length increases and instead of having a sigma bond and one or more pi bonds al that remains is a M-M single bond with an nonbonded electron pair (donor) on one metal center leaving the other center electron-deficient with a positive charge. Some of these compounds break up in some kind of Wanzlick equilibrium to form the corresponding carbenes which also have acceptor and donor sites.
Thus armed with occupied and empty frontier orbitals these compounds display reactivity not unlike that of transition metals for example with small molecules such as hydrogen and ethylene. The compound ArGe:::GeAr reacts with hydrogen in oxidative addition (ordinarily the preserve of transition metals) owing to electron donation of the occupied hydrogen sigma bond into the symmetrical GeGe LUMO on the one hand and back donation of the GeGe pi bond into the antibonding hydrogen sigma bond. Carbenes and the heavier analogs also react with hydrogen and also occasionally with ammonia Frustrated Lewis pairs too owe their reactivity to favorable non-self extinguishing frontier orbitals. In addition many compounds in this cluster coordinate with alkenes just like transition metals in the Dewar-Chatt-Duncanson model.
Power in his review presents a strong case for main group element as transition elements which is also the title but is the theory entirely new? The isolobal principle proposed by Roald Hoffmann in 1976 at least also explains reactivity by looking at frontier orbitals only for example between that of the methyl (main group carbon) radical and manganese (transition metal) pentacarbonyl.
Power, P. (2010). Main-group elements as transition metals Nature, 463 (7278), 171-177 DOI: 10.1038/nature08634