<P> Tetrahedral coordination is a major structural motif in silicon chemistry just as it is for carbon chemistry . However, the 3p subshell is rather more diffuse than the 2p subshell and does not hybridise as well with the 3s subshell . As a result, the chemistry of silicon and its heavier congeners shows significant differences from that of carbon, and thus octahedral coordination is also significant . For example, the electronegativity of silicon (1.90) is much less than that of carbon (2.55), because the valence electrons of silicon are further from the nucleus than those of carbon and hence experience smaller electrostatic forces of attraction from the nucleus . The poor overlap of 3p orbitals also results in a much lower tendency towards catenation (formation of Si--Si bonds) for silicon than for carbon due to the concomitant weakening of the Si--Si bond compared to the C--C bond: the average Si--Si bond energy is approximately 226 kJ / mol, compared to a value of 356 kJ / mol for the C--C bond . This results in multiply bonded silicon compounds generally being much less stable than their carbon counterparts, an example of the double bond rule . On the other hand, the presence of 3d orbitals in the valence shell of silicon suggests the possibility of hypervalence, as seen in five - and six - coordinate derivatives of silicon such as SiX and SiF . Lastly, because of the increasing energy gap between the valence s and p orbitals as the group is descended, the divalent state grows in importance from carbon to lead, so that a few unstable divalent compounds are known for silicon; this lowering of the main oxidation state, in tandem with increasing atomic radii, results in an increase of metallic character down the group . Silicon already shows some incipient metallic behaviour, particularly in the behaviour of its oxide compounds and its reaction with acids as well as bases (though this takes some effort), and is hence often referred to as a metalloid rather than a nonmetal . However, metallicity does not become clear in group 14 until germanium and dominant until tin, with the growing importance of the lower + 2 oxidation state . </P> <P> Silicon shows clear differences with carbon . For example, organic chemistry has very few analogies with silicon chemistry, while silicate minerals have a structural complexity unseen in oxocarbons . Silicon tends to resemble germanium far more than it does carbon, and this resemblance is enhanced by the d - block contraction resulting in the size of the germanium atom being much closer to that of the silicon atom than periodic trends would predict . Nevertheless, there are still some differences because of the growing importance of the divalent state in germanium compared to silicon, that result in germanium being significantly more metallic than silicon . Additionally, the lower Ge--O bond strength compared to the Si--O bond strength results in the absence of "germanone" polymers that would be analogous to silicone polymers . </P> <P> Many metal silicides are known, most of which have formulae that cannot be explained through simple appeals to valence: their bonding ranges from metallic to ionic and covalent . Some known stoichiometries are M Si, M Si, M Si, M Si, M Si, M Si, M Si, M Si, M Si, MSi, M Si, MSi, MSi, and MSi . They are structurally more similar to the borides than the carbides, in keeping with the diagonal relationship between boron and silicon, although the larger size of silicon than boron means that exact structural analogies are few and far between . The heats of formation of the silicides are usually similar to those of the borides and carbides of the same elements, but they usually melt at lower temperatures . Silicides are known for all stable elements in groups 1--10, with the exception of beryllium: in particular, uranium and the transition metals of groups 4--10 show the widest range of stoichiometries . Except for copper, the metals in groups 11--15 do not form silicides . Most instead form eutectic mixtures, although the heaviest post-transition metals mercury, thallium, lead, and bismuth are completely immiscible with liquid silicon . </P> <P> Silicides are usually prepared by direct reaction of the elements . For example, the alkali metals and alkaline earth metals react with silicon or silicon oxide to give silicides . Nevertheless, even with these highly electropositive elements true silicon anions are not obtainable, and most of these compounds are semiconductors . For example, the alkali metal silicides (M) (E) contain pyramidal tricoordinate silicon in the Si anion, isoeelctronic with white phosphorus, P. Metal - rich silicides tend to have isolated silicon atoms (e.g. Cu Si); with increasing silicon content, catenation increases, resulting in isolated clusters of two (e.g. U Si) or four silicon atoms (e.g. (K) (Si)) at first, followed by chains (e.g. CaSi), layers (e.g. CaSi), or three - dimensional networks of silicon atoms spanning space (e.g. α - ThSi) as the silicon content rises even higher . </P>

What type of bonding is present in si