Trigonal pyramid orbitals

Let s look at an example. In ammonia (NH3), the nitrogen atom is sp hybridized, so all four orbitals arrange in a tetrahedral structure, just as we would expect. But only three of the orbitals in this arrangement are responsible for bonds. So, if we look just at the atoms that are connected, we do not see a tetrahedron. Rather, we see a trigonal pyramidal arrangement ... 

This nitrogen atom has three bonds and one lone pair, so it is sp hybridized, just as we would expect. The lone pair occupies an sp hybridized orbital, and the nitrogen atom has trigonal pyramidal geometry, just as we saw in the previous section. But now consider the nitrogen atom in the following compound ... 

Inspection of the second resonance structure reveals that this nitrogen atom is actually sp hybridized, not sp. It might look like it is sp hybridized in the first resonance structure, but it isn t. Here is the general rule a lone pair that participates in resonance must occupy ap orbital. In other words, the nitrogen atom in the compound above is sp hybridized. And as a result, this nitrogen atom is trigonal planar rather than trigonal pyramidal. 

Remember that the molecular shape ignores the lone pair. The hydronium ion has a trigonal pyramidal shape described by the three s p hybrid orbitals that form bonds to hydrogen atoms. 

In their pursuit of modeling Type I copper proteins, Kitajima et al. reported112 a rare, tetrahedrally coordinated complex (105), which displayed an EPR spectrum consistent with the presence of the unpaired electron in the dz2 orbital.1 They also isolated a square-pyramidal DMF adduct (complex (106)). They were successful in providing structural proof of a copper(II) complex (trigonal pyramidal) with C6F5S -coordinated complex (107), with CuN3S chromo-phore.113 The X-ray analysis (poor data set) of a closely similar complex with Ph3CS as the... 

Bismuth phosphine complexes represent a substantial component of the established phosphine complexes of heavier p-block elements, and an excellent overview has presented an important bonding model for these systems (7). The observed structures are considered as trigonal-pyramidal BiX3 units with three secondary trans bonds. If the acceptor orbitals are the Bi-X trans arrangement is expected, as the relationship between the trans X-Bi-P bond distances. The shortest Bi-P distance [2.7614(2) vs 2.866(3) A] is trans to the longer Bi-Br distance [3.403(1) vs 2.9916(1) A], as the only arrangement that will allow the phosphine ligands to occupy trans... 

The ammonia molecule is a trigonal pyramid, belonging to the C3v point group. The 2s and 2p orbitals of the nitrogen atom and the Is orbital group combinations of the three hydrogen atoms transform, with respect to the C3v point group, as indicated in Table 6.1. 

The three Is atomic orbitals of the trigonally pyramidal C3v NH3 molecule have the character given below ... 

For each of the following molecules, determine what atomic orbitals on the central atom are allowed by symmetry to be used in the construction of sigma hybrid orbitals, a. NHj (trigonal pyramid) b. BF3 (trigonal plane) c. SF6 (octahedron) d. PF, (trigonal bipyramid)... 

Formulate the bonding in NH2 in terms of delocalized molecular orbitals. The molecule is trigonal-pyramidal (C3V point group). Compare the general molecular-orbital description with a localized tetrahedral model for NH3. Discuss the values of the following bond angles H—N—H, 107° H— P—H (in PH2), 94° and F—N —F (in NF3 ), 103 °. 

The most common valence states of arsenic are —3, 0, +3, and +5 (Shih, 2005), 86. The —3 valence state forms through the addition of three more electrons to fill the 4p orbital. In the most common form of elemental arsenic (As(0)), which is the rhombohedral or gray form, each arsenic atom equally shares its 4p valence electrons with three neighboring arsenic atoms in a trigonal pyramid structure ((Klein, 2002), 336-337 Figure 2.1). The rhombohedral structure produces two sets of distances between closest arsenic atoms, which are 2.51 and 3.15 A (Baur and Onishi, 1978), 33-A-2. The +3 valence state results when the three electrons in the 4p orbital become more attracted to bonded nonmetals, which under natural conditions are usually sulfur or oxygen. When the electrons in both the 4s and 4p orbitals tend to be associated more with bonded nonmetals (such as oxygen or sulfur), the arsenic atom has a +5 valence state. 

Presumably the differences observed in the H-3,P coupling in the series of compounds shown in Figure 4 can be attributed to the hybridization of the phosphorus atom. When the phosphorus is trigonal pyramidal (PR3) the hybridization of the phosphorus atom is essentially p3, with the s orbital primarily located on the lone pair. However, when the phosphorus becomes four coordinate (i.e. pseudotetrahedral) the hybridization changes to approximately sp3. Thus, since coupling information is communicated via the s elections, the more s character a bond has the higher the coupling between the atoms, and this series of compounds demonstrates this. 

Because the 2p orbitals are mutally perpendiculai the simple VB model predicts a trigonal pyramid with angles of 90 degrees. 

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