Strength of Bonds Involving sp Hybridization COLIN T. MORTIMER University College of North Staffordshire, England
D u m w the last ten years a ronsiderable amount of data has been accumulutcxd about the strength of metal-carbon bonds. For example, in the e~rbonyl compounds of the transition metals the bonding cousists of a r-bond supplemented by n-bonding (1). On the other hand, the alkyls of beryllium, which in the dimer molecules are electron deficient, oont.ain "methyl bridges" between beryllium atoms and these metal-carbon "half-bonds" are weaker than the normal, single covalent bonds (2). In the metal-eyalopn~t,adienyl compounds such as ferrocene, where t.he iron atom is "sandwirhed" bebeen two cyclopentadienyl rings, each ring must be cousidered as a whole and the molecular orbital treatment results in a single covalent bond from the riug as a whole, not from particular carbon atoms, but "resonating" equally among all five carbon atoms. Metal-carbou bonds which are uot complicated by such problems, and which therefore provide a simpler starting point, are those found in the alkyl compounds of the non-transition metals, with an underlying shell of 18 electrons, i.e., Zn, Cd, Hg; Gn, In, TI; Ge, Sn, Ph; and As, Sb, Bi.
k n o w data on t.he paraffins. The dissociation energies, D(K-H) have been measured, mainly by using the elert,ron impact method (see for example Stevenson (.I)), and the heats of formation of the paraffins and of :~tornic hydrogen are well established. Substituting these in the equation,
EXPERIMENTAL DETERMINATIONS
As the heat of reactiou is t.he difference between the heats of formation of the products and reactants, if t,his is measured and the heat,s of formation of methyl iodide, iodine and zinc iodide are known, that of the einr diethyl can be calculated. Although this approach gives the total dissociation energy, which may be divided among the n bonds, to give a mean dissociation energy, it gives no information about the actual heat. required t,o remove the first or subsequent R groups, i.e., D l , Dl, et.c., for it is usually t,hecase that D f D, f D,. I n the second method of dekrmining bond dissocia tion energies, the molecule is dissociated into its fragments, usually by pyrolysis. The kinetics are used to calculate the velocity constant and the activation energy of the decomposition reaction. This method has been applied to a number of organo-mercury compounds, HgR2, and Warhurst (8) has found that these fall into t,wo classes, according t,o t,he type of decomposition. The first group contaius those metal alkyls which dissociate to lose one R group, i.e., HgRl HgR R. The activation energy of t,his reaction is taken as the dissociation energy, Dl. of t,hefirst mercury-carbon bond. On the other hand there are t.hose rompounds in which it is supposed that the wtivation energy is not localized in one bond, hut i l l I ~ t,heI metal-carbon bonds,
The determination of bond strengths in these compounds has been approached in two different, hut. complementary, ways. The first method uses the heat of formation of the gaseous molecule, AH,'(gas). In an organometallic molecule, MR,
