Some energetic and bonding considerations (group 14 elements)

Table 1.1 lists some experimentally determined values for covalent bond enthalpy terms.

    When we try to interpret the chemistry of the group 14 elements on the basis of such bond energies, caution is necessary for two reasons:

• many thermodynamically favourable reactions are kinetically controlled;
• in order to use bond enthalpy terms successfully, complete reactions must be considered.

Table 1.1 Some experimental covalent bond enthalpy terms (kJ mol-1); the values for single bonds refer to the group 14 elements in tetrahedral environments.

    The first point is illustrated by considering that although the combustions of CH₄ and SiH₄ are both thermodynamically favorable, SiH₄ is spontaneously inflammable in air, whereas CH₄ explodes in air only when a spark provides the energy to overcome the activation barrier. In respect of the second point, consider reaction 1.1.

           (1.1)

Inspection of Table 1.1 shows that E(C^_H) > E(C^_Cl), but the fact that the H^_Cl bond (431 kJ mol^-1) is significantly

stronger than the Cl^_Cl bond (242 kJ mol^-1) results in reaction 1.1 being energetically favourable. Catenation is the tendency for covalent bond formation between atoms of a given element, e.g. C^_C bonds in hydrocarbons or S^_S bonds in polysulfides.

   The particular strength of the C^_C bond contributes towards the fact that catenation in carbon compounds is common. However, it must be stressed that kinetic as well as thermodynamic factors may be involved, and any detailed discussion of kinetic factors is subject to complications:

• Even when C^_C bond breaking is the rate-determining step, it is the bond dissociation energy (zero point energy) rather than the enthalpy term that is important.
• Reactions are often bimolecular processes in which bondmaking and bond-breaking occur simultaneously, and in such cases, the rate of reaction may bear no relationship to the difference between bond enthalpy terms of the reactants and products.

    In contrast to the later elements in group 14, C tends not to expand its valence octet of electrons, and, while complexes such as [SiF₆]^2_ and [Sn(OH)₆]^2- are known, carbon analogues are not. The fact that CCl₄ is kinetically inert towards hydrolysis but SiCl₄ is readily hydrolysed by water has traditionally been ascribed to the availability of 3d orbitals on Si, which can stabilize an associative transition state. This view has been challenged with the suggestion that the phenomenon is steric in origin associated purely with the lower accessibility of the C centre arising from the shorter C^_Cl bonds with respect to the Si^_Cl bonds. On the other hand, (p–p) π-bonding leading to double to triple homonuclear bonds, which is so common in carbon chemistry, is relatively unimportant later in the group. A similar situation is observed in groups 15 and 16. The mesityl derivative 1.1 was the first compound containing an Si=Si bond to be characterized; in the Raman spectrum, an absorption at 529cm^-1 is assigned to the ν (Si=Si) mode, and in the solid state structure, the Si^_Si bond distance of 216 pm is less than twice the value of rcov (2 × 118 pm).

( 1.1)

Such species are stabilized with respect to polymerization by the presence of bulky substituents such as mesityl (in 1.1), CMe₃ or CH(SiMe₃)₂. The central Si₂C₄-unit in 1.1 is planar, allowing overlap of orthogonal 3p orbitals for π-bond formation; the bulky mesityl substituents adopt a ‘paddle-wheel’ conformation minimizing steric interactions.† In contrast, theoretical studies on Si₂H₄ (mass spectrometric evidence for which has been obtained), indicate that the non-planar structure is energetically favoured. The same trans-bent conformation has been observed experimentally for Sn₂R₄ compounds . Silicon–silicon triple bonds remain unknown. Theoretical studies on the hypothetical HSi≡SiH suggest that a non-linear structure is energetically preferred over an ethyne-like structure. Experimental efforts to realize the Si≡Si bond continue .

The formation of (p–p)π-bonds between C and Si is also rare; an example is shown in equation 1.2. In 1999, the first examples of a C^_Si bond were confirmed in the gasphase molecules HC≡SiF and HC≡SiCl. These species were detected using neutralization–reionization mass spectrometry, but have not been isolated.

     (1.2)

The first Ge=C double bond was reported in 1987, since when a number of examples have been reported, including Mes₂Ge=CHCH₂ ^tBu which is stable at 298 K.