Chemical Bonding in Higher Main Group Elements

Abstract

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, 0x0 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.

Construction of Building Blocks for Highly Functional π-Electron Silicon Nanomaterials by First-Principles Calculations

“…Nowadays, the following explanation has been vindicated for the difference in geometry between the unsaturated carbon and silicon systems (Kutzelnigg 1984): the essential difference between the atoms of the fi rst and higher rows is that the cores of the former contain only s-atomic orbitals, whereas the cores of the latter include at least s-and p-atomic orbitals. The s and p valence atomic orbitals of fi rst row atoms are localized in roughly the same region of space, while the p valence atomic orbitals of higher row atoms are much more extended in space.…”

Phosphorus‐Containing Polycyclic Aromatic Hydrocarbons

“…The situation for compounds with s 3 ,l 3 -P is complex. Because isovalent hybridisation is not favoured for heavier elements, [26] the lone pairo fp hosphorus retains significant sc haracter (consequently, s 3 ,l 3 -P has al arge barriert op yramidalisation), and conjugationw ith the p system is reduced. With enforced planarity of s 3 ,l 3 -P, [27] an excellent conjugative building block can be formed.…”

Polyanionic Hexagons: X6n– (X = Si, Ge)

“…A common explanation for the difference in geometry between carbon and the heavier Group 14 elements was that the energy separation between valence s and p orbitals is small only in first-row elements, allowing efficient hybridization, and the energy separation in higher rows is much larger. Kutzelnigg revealed, however, that the energy separation between s and p orbitals is rather small in heavy atoms from the experimental data of s-p excitation energy with conservation of spin multiplicity [9]. Nowadays, the following explanation has been vindicated: the essential difference between the atoms of the first and higher rows is that the cores of the former contain only s-atomic orbitals, whereas the cores of the latter include at least s-and patomic orbitals.…”