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Carbon Group (Group 4) Trends

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  • Summary of Carbon Group (Group IVA) Trends:

    1. Stabilization of (+2) oxidation state relative to (+4) of the elements down the group.

    The halides, oxides, and sulfides of the M2+ ions become more stable on descending the group. For example SiCl4, SiBr4, and SiI4 are all stable. PbCl4 decomposes at 105 °C and PbI4 does not exist. Similarly the ease of oxidation of the M2+ halides decreases down the column.
    PbCl2 may be converted to PbCl4 by heating in a stream of chlorine.
    Similarly PbO2 is an oxidizing agent, whereas SnO2, GeO2, and SiO2are not.
    The stabilities of the MII and MIV organometallic derivatives of the elements behave differently. Pb(C2H5)4 can be readily stored and is more stable than Pb(C2H5)2, which is not isolable as a solid.
    The compounds in the lower oxidation state are in general more ionic, less likely to form molecular structures, the halides are less readily hydrolyzed and the oxides are less acidic.

    2. Hydrides and alkyls become less stable down the column.

    The M-H and M-C mean bond enthalpies decrease down the column and consequently the hydrides and alkyls become thermodynamically less stable and kinetically more reactive. Carbon, of course, forms a very wide range of hydrides, silicon forms primarily SiH4 and Si2H6 which are spontaneously inflammable. Higher silanes decompose readily to Si2H6. The Si-H bond polarities are opposite to C-H.
    Silanes are strong reducing agents. The germanes GeH4, Ge2H6, and Ge3H8 are less flammable than SiH4 and are resistant to hydrolysis. SnH4 decomposes at 0°C to Sn and PbH4 is extremely unstable.
    The organometallic derivatives of silicon and germaniu are very similar. They are more reactive than the carbon analogues because the M-C bonds are more polar and the central atom can exand its coordination number more easily. The rates of hydrolysis are in order:
    Pb >> Sn >> Ge >Si.
    Organotin compounds more readily expand their coordination geometries and more readily form cationic species. Organolead compounds decompose readily at 100-200 °C by free radical processes.

    3. Catenation

    The element-element mean bond enthalpies decrease in the order:
    C-C > Si-Si > Ge-Ge > Sn-Sn > Pb-Pb
    and therefore the range of ring and polyhedral molecules diminish down the group. Carbon not only forms an extensive range of chain and ring compounds, but also polyhedral molecules such as prismanes, C6H6, and cubane (C8H8). Analogous compounds are known for Si, Ge, and Sn if the hydrogens are replaced by bulky organic substituents.
    However, few examples exist for Pb, which form compounds containing the anionic Zintl polyhedral anoin Pb94-analogous to Sn52-.

    4. Multiply bonded compounds

    The ability of the elements to form multiple bonds diminishes in the series:
    C-C > Si-Si > Ge-Ge > Sn-Sn > Pb-Pb
    Because the pπ-pπ overlapsbecome less favorable. This has the following manifestations:
    a) The elements below C the allotropes which would structurally resemble graphite which has a delocalized two dimensional π-system are not observed.
    b) There are no simple analogues of ethane (C2H4) and ethyne (C2H2) and compounds of SI, Ge, and Sn with multiple bonds may only be isolated when there are bulky organic substituents on the group IV atoms. Furthermore, the Ge and Sn compounds do not have planar geometries.
    c) The analogues of CO2 and CS2 have polymeric structures rather than triatomic molecular geometries.
    d) The heavier elements do not form analogues of carbides with C22- and C32- multiply bonded ions.

    5. Elements become progressively more metallic down the column. Cabon is especially in its diamond and polyhedral forms is a typical non-metal, silicon is a semiconductor, and tin & lead are typical metals. Although tin has one modification (grey tin) which is isostructural with Ge, Si, and diamond. Lead only occurs in close packed structural forms.

    6. The oxides become more basic down the column.

    CO2 and SiO2 are acidic oxides, SnO2 is amphoteric, and GeO2 is mainly acidic with slight amphoteric character. The Si-O mean bond enthalpies are particularly large and this leads to a wide range of silicates. In general for the elements below carbon the M-O bonds are sufficiently strong that the oxides are susceptible to hydrolysis.

    7. The typical coordination numbers increase down the group.

    For carbon the tetradedral geometry predominates unless multiple bonds formed. For the havier elements the tetrahedral geometry is also widespread bu the larger size of the central atoms leads to the formation of compounds with higher coordination numbers, e.g.
    SiF5-, Trigonal bipyramidal; SiF62-, Octahedral; SnCl5-, Trigonal bipyramidal; Sn(C6H5)(NO3)2(OP(C6H5)3, Pentgonal bipyramidal (7 coord); Sn(NO3)4, Dodecahedral (8 coord)
    The increased facility to achieve the higher coordination numbers is also reflected in the transition from molecular to ppolymeric, e.g. CF4, SiF4, and GeF4 ar e molecular, where as SnF4 amnd PbF4 have infinite lattices based on octahedral metal centers.
    Of course all of these compounds with coordination numbers is also reflected in the geometries of the oxo-anions:
    trig. planar: CO32- tetrahedral: SiO42- octahedral: Ge(OH)62- Sn(OH)62- Pb(OH)62-
    The chlorides of Ge, Sn, and Pb react with aqueous HCl to form the [MCl6]2- anions, where as SiCl4 hydrolyses and CCl4 is unreactive. However, SiF4 does form [SiF6]2- with HF.

    8. The heavier elements form a wider range of complexes and more cationic complexes. Si, Ge, Sn, and Pb all form oxalato-complexes [M(ox)3]2- and cationic complexes [M(acac)3}+.

    9. Ease of reduction of halides.

    Although C-Cl and Ge-Cl bonds in four valent compounds are reduced to the corresponding hydrides by Zn and HCl, Si-Cl and Sn-Cl bonds are not.

    10. SiH4 hydrolyses in the presence of trace amounts of base more readily than CH4, GeH4, and SnH4.