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CGMT model
The Carter Goddard Malrieu-Trinquier model (CGMT model) is used to predict the geometries of multiply bonded main group elements. The model gives simple estimates of the stabilities of planar and trans bent structures. A double bonded molecule, with a sigma and a pi bond, is considered to be derived from two carbene (or carbene analog) units that have two unpaired electrons, that is they are in a triplet state. A planar structure is expected when the overall bond energy (sigma plus pi bond) is greater than that of the promotion energy of the two carbene fragments from singlet to triplet state. If the overall bond energy is less than the promotion energy of the two carbene fragments from singlet to triplet state a trans bent structure, with two dative bonds is predicted. The singlet form becomes progressively more stable than the triplet form on going down the group. The energy gained by the formation of an alkene like planar sigma and pi system becomes inceasingly unable to offset the energy required to form the triplet state from the singlet state. Expressed as follows:


 * Esigma+pi = Edouble bond - 2ΔEsinglet→triplet

If Esigma+pi > 2ΔEsinglet→triplet, a planar structure with alkene like bonding results, if Esigma+pi < 2ΔEsinglet→triplet, a trans/bent structure with two dative bonds results, if Esigma+pi < ΔEsinglet→triplet, then there will be no bond formed.

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. 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 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. The Carter-Goddard-Malrieu-Trinquier model can be used to predict the nature of the bonding.

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.

Examples of heteronuclear bonds
table?? RColumns Si,Ge,Sn, Pb Rows C Si, Ge, etc

Double bonds to the heavier group 15 and 16 elements
Many compounds are known for heavier group 15 and 16 elements which are described as having which involve expansion of the octet. Exaples are


 * Table with POCl3, PO43-, SO3, DMSO

The bonding scheme proposed by Pauling was that there was overlap between the empty d orbitals on the central atom and the full p orbitals on for example oxygen, forming a  dpi-ppi bond. These E=O bonds are generally shorter than the E–O.

See Corbridge phosphorus for discussion on P=O and P=S bonds and the evidence for double bonding character.

double bonds Oxygen forms compounds with all of the heavier group 15 and 16 elements. Many of these are described as having double bonds to oxygen, and many of these involve the group 5 or group 16 element violating the octet rule. This expansion of the octet was believed to be achieved by the involvement of empty d orbitals, the double bond bond being formed by a lone pair in a p orbital on oxygen overlapping with empty d orbitals on the group 15 or 16 elemnt.( pπ - dπ bonding)

double bonds to group 14
Silanone

double bonds to group 15
N,P,As, Sb, Bi

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