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Group 14 alkene homologs

Double bonded compounds, alkene homologs, R₂E=ER₂ are now known for all of the heavier group 14 elements. These are not planar like the alkenes but adopt twisted and/or trans bent structures. These effectss become more pronounced for the heavier elements. The distannene (Me₃Si)₂CHSn=SnCH(SiMe₃)₂ has a tin-tin bond length just a little shorter than a single bond, a trans bent structure with pyramidal coordination at each tin atom, and readily dissociates in solution to form (Me₃Si)₂CHSn: (stannanediyl, a carbene analogs. The bonding comprises of two weak donor acceptor bonds, the lone pair on each tin atom overlapping with the empty p orbital on the other.^[2][3] In contrast, in disilenes each silicon atom has planar coordination but the substituents are twisted so that the molecule as a whole is not planar. In diplumbenes the Pb=Pb bond length can be longer than that of many corresponding single bonds^[3] Plumbenes and stannenes generally dissociate in solution into monomers with bond enthalpies that are just a fraction of the corresponding single bonds. Some double bonds plumbenes and stannes are similar in strength to hydrogen bonds.^[4] The Carter-Goddard-Malrieu-Trinquier model can be used to predict the nature of the bonding.^[5]

ne way in which the bonding is decribed. This model considers the energetics of homolytic double bond breakage forming triplet carbene like fragments with two unpaired electrons, and takes into account the ground state of the group 14 carbene analogs which have a singlet ground state with no unpaired electronsin contrast to carbon which has a triplet state with two unpaired electrons,  whereas the heavier elemnts the carbene analogues have a singelt groud stae with  two paired electrons.

MT model (CGMT stands for )

Si=Si Ge=Ge Sn=Sn Pb=Pb

from kutzelnigg Angewandte Chemie International Edition in English Volume 23, Issue 4, pages 272–295, April 1984 Many concepts used for a qualitative description of chemical bonding that originated in the early days of theoretical chemistry have been vindicated recently by quantum chemical calculations, at least as far as first row elements are concerned. However, many concepts that have been justified for first row elements (Li to Ne) cannot—contrary to widespread belief—be generalized to the higher main group elements. This applies particularly to the concept of hybridization, which should be viewed with considerable caution. The essential difference between the atoms of the first and higher rows is that the cores of the former contain only s-AOs, whereas the cores of the latter include at least s- and p-AOs. As a consequence, the s and p valence AOs of first row atoms are localized in roughly the same region of space, while the p valence AOs of higher row atoms are much more extended in space. This has the consequence that for the light main group elements both lone-pair repulsion and isovalent hybridization play a greater role than for the heavy main group elements. Furthermore, this implies that single bonds between first row elements are weak and multiple bonds are strong, whereas for the second or higher row elements single bonds are strong and multiple bonds weak. The “extended valence” (violation of the octet rule) observed in compounds of higher main group elements has very little to do with the availability of d-AOs but is due rather to the size of these atoms and thus to the reduced steric hindrance between ligands and, to a lesser extent, also to the lower electronegativity of the heavy atoms. A model based on the concept of electron-rich multicenter bonds is certainly closer to reality than one involving hybrids with the participation of d-AOs. The XO bonds in phosphane oxides, sulfoxides, oxo acids and related compounds are better formulated as semipolar rather than as true double bonds, even if they behave in some respects like double bonds.—The growing interest of theorists in compounds of higher main group elements parallels new and, in some instances, spectacular results of experimental research on the chemistry of these elements.

Group 15, E=E double bonds

Group 16 homonuclear E=E bonds

Double bonds involving oxygen

double bonds

double bonds to group 15=

N,P,As, Sb, Bi

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