Central atoms, expanded

Many molecules with central atoms from Period 3 or higher take part in Lewis acid-base reactions in which the central atom expands its valence shell. SnCl4 reacts with (CH3)3N as follows ... 

As you can see, the central atoms in these molecules have expanded octets. In PC15, the phosphorus atom is surrounded by 10 valence electrons (5 shared pairs) in SF6, there are 12 valence electrons (6 shared pairs) around the sulfur atom. 

Sometimes, as with PCI5 and SF6, it is clear from the formula that the central atom has an expanded octet Often, however, it is by no means obvious that this is the case. At first glance, formulas such as C1F3 or XeF4 look completely straightforward. However, when you try to draw the Lewis structure it becomes clear that an expanded octet is involved. The number of electrons available after the skeleton is drawn is greater than the number required... 

In many expanded-octet molecules, one or more of the electron pairs around the central atom are unshared. Recall, for example, the Lewis structure of xenon tetrafluoride, XeF4 (Example 7.4). 

Molecular geometries for molecules with expanded octets and unshared electron pairs. The gray spheres represent terminal atoms (X), and the open ellipses represent unshared electron pairs (E). For example. AX4E represents a molecule in which the central atom is surrounded by four covalent bonds and one unshared electron pair. 

In each of the following polyatomic ions, the central atom has an expanded octet. Determine the number of electron pairs around the central atom and the hybridization in... 

Expanded octet More than four electron pairs about a central atom, 173-174 and hybridization, 187 and molecular geometry, 181 Expansion, 339-340... 

The octet rule tells us that eight electrons fill the outer shell of an atom to give a noble-gas ns1ns(l valence-shell configuration. However, when the central atom in a molecule has empty d-orbitals, it may be able to accommodate 10, 12, or even more electrons. The electrons in such an expanded valence shell may be present as lone pairs or may be used by the central atom to form additional bonds. 

When different resonance structures are possible, some giving the central atom in a compound an octet and some an expanded valence shell, the dominant resonance structure is likely to be the one with the lowest formal charges. However, there are many exceptions and the selection of the best structure often depends on a careful analysis of experimental data. 

The various types of Lewis acids are protons, simple cations, electron-deficient molecules, compounds in which the central atom can expand its octet, and elements with an electron sextet. 

Hydrogen never has an octet of electrons in any of its compounds, but rather a pair (or duet, if you prefer). An example is the Lewis structure of H20 (below). In many compounds in which the central atom is from the second period or higher, there are more than eight electrons around the central atom an example of a compound with such an expanded octet is IC13 (below). Finally, in some compounds, there are less than eight electrons around the central atom one such electron deficient compound is BF3. 

Central Atoms with an Expanded Valence Level... 

This polyatomic ion has six bonds around the central Si atom, an obvious exception to the octet rule, so the central atom needs an expanded valence shell. 

Keep in mind that the need for an expanded valence level for the central atom may not always be as obvious as in the previous Sample Problem. For example, what if you were asked to predict the molecular shape of the polyatomic ion, BrCU" Drawing the Lewis structure enables you to determine that the central atom has an expanded valence level. 

There are not enough electrons for bonding. Therefore, the central atom requires an expanded valence level. A possible Lewis structure for BrCU is ... 

Draw Lewis structures for the following molecules and ions, and use VSEPR theory to predict the molecular shape. Indicate the examples in which the central atom has an expanded octet. 

Draw and compare the Lewis structures of CIO4 and OSCI2. In which of these cases, if any, does the central atom have an expanded valence energy level ... 

The original concepts of metal-ligand bonding were essentially related to the dative covalent bond the development of organometallic chemistry has revealed a further way in which ligands can supply more than one electron pair to a central atom. This is exemplified by the classical cases of bis(benzene)chromium and bis(cyclopentadienyl)iron, trivial name ferrocene. These molecules are characterised by the bonding of a formally unsaturated system (in the organic chemistry sense, but expanded to include aromatic systems) to a central atom, usually a metal atom. 

Because the additional electrons must be accommodated in valence orbitals, only nonmetal atoms in Period 3 or higher can expand their octets. These elements have empty ci-orbitals in the valence shell. Another factor—possibly the main factor—in determining whether more atoms than allowed by the octet rule can bond to a central atom is the size of that atom. A P atom is big enough for up to six Cl atoms to fit comfortably around it, and PC1S is a common laboratory chemical. An N atom, though, is too small, and NC15 is unknown. 

Here wi and w2 are the positive elastic springs (wli2 > 0), qh 1=1, 2 is the symmetrized difference of the central displacements of the central atom and its nearest neighboring atoms, i denotes the row of the -representation, Qni are all the other displacements of the crystal, being orthogonal to q and q2,1 is the second-order unit matrix, configurational coordinates q and q2 can be expanded into the normal coordinates xn and x2j of the -representation as follows ... 

You are attracted to the central atom, but there s nothing on your other side, so you are free to expand in that direction. That expansion means that you take up more room than the other electron pairs, and they are all squeezed a little closer together because of you. Multiple bonds have a similar effect because more space is required for more electrons. In general, unshared electron pairs and multiple bonds decrease the angles between the remaining bonds. A few examples are shown in the following tables. 

The octet rule applies quite well to the first full row of the periodic table (Li through F), but beyond this it is generally applicable only to the non-transition elements, and even in many of these it cannot explain many of the bonding patterns that are observed. The principal difficulty is that a central atom that is bonded to more than four peripheral atoms must have more than eight electrons around it if each bond is assumed to consist of an electron pair. In these cases, we hedge the rule a bit, and euphemistically refer to the larger number of electrons as an expanded octet . 

Examples of molecules in which the central atom contains an expanded octet are the phosphorus pentahalides and sulfur hexafluoride. 

If the central atom possesses partially occupied d -orbitals, it may be able to accommodate five or six electron pairs, forming what is sometimes called an expanded octet . 

We initially believed [23] that a similar mode of description carries over to 03, but we now realise that this system is somewhat more complicated than we had first supposed. Calculations for the four out-of-plane n electron, whether using spin-coupled theory or at the CASSCF level, reveal that there exist two solutions that are remarkably close in energy. The one that lies lowest, provided we optimize properly the description of the inactive electrons, corresponds to a singlet diradical whereas the other corresponds to a hypercoordinate central atom [24]. It is clear that neither description carries much conviction on its own, and that we must consider expanding the active space. 

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