Trigonal bipyramidal orbitals

Physical Properties. Sulfur tetrafluoride has the stmcture of a distorted trigonal bipyramid, the sulfur having hybrid sp d orbitals and an unshared electron pair (93). The FSF bond angles have been found to be 101° and 187°, and the bond distances 0.1646 and 0.1545 nm (94). 

Calculations for trigonal bipyramidal ML4(NO) systems with axial NO-like [Ir(NO)(PPh3)3H+] give a d orbital sequence of xz,yz < x2 — y2, xy < z2 so that in such an IrNO 8 system, the z2 orbital is unoccupied not only does bending not produce any stabilization but in fact dxz, dyz — 7r back-bonding is lost, favouring a linear Ir—N—O bond. 

The orientation of bonds on a central atom of a molecule that has no lone pairs can be any of those listed below. What is the hybridization of the orbitals used by each central atom for its bonding pairs (a) tetrahedral (b) trigonal bipyramidal ... 

An inner atom with a steric number of 5 has trigonal bipyramidal electron group geometry and can be described using s p d hybrid orbitals. 

With a steric number of 5, chlorine has trigonal bipyramidal electron group geomehy. This means the inner atom requires five directional orbitals, which are provided hymsp d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the chlorine atom. Remember that the trigonal bipyramid has nonequivalent axial and equatorial sites. As we describe in Chapter 9, lone pairs always occupy equatorial positions. See the orbital overlap view on the next page. 

Fig. 15. The overlap of (ligand) p orbital with a vacant orbital upon rearrangement of the trans compound to a trigonal bipyramid.

These compounds are often referred to as hypervalent. The apical bonds in a trigonal bipyramid are described by molecular orbitals constructed from a p-orbital on the central atom and o-bonding orbitals (p- or sp hybrid) on the apical ligands. The molecular orbitals can be drawn as ... 

In order to correctly predict which ligands occupy which sites in such compounds, one must recognize that, as a general rule, fluorines will always prefer the axial site in a trigonal bipyramidal system, perhaps because of fluorine s small size, but probably also because of its preference to bind to orbitals with as little s-character as possible. The orbitals used by P to make its axial bonds have less s-character than those used to make its equatorial bonds. This is reflected by the larger F—P coupling constants to the equatorial fluorine substituents. 

Figure 2. Diagrams showing the directions of the maxima of octahedral sp3d2 and trigonal bipyramidal and square pyramidal sp3d hybrid orbitals.

A simple symmetry-based analysis of the bonding orbitals for the carbene and metal fragments of a trigonal bipyramidal d8 complex suggests that the vertical orientation (i) should be preferred (100) ... 

Perhaps the most fruitful of these studies was the radiolysis of HCo(C0)4 in a Kr matrix (61,62). Free radicals detected in the irradiated material corresponded to processes of H-Co fission, electron capture, H-atom additions and clustering. Initial examination at 77 K or lower temperatures revealed the presence of two radicals, Co(C0)4 and HCo(C0)4 , having similar geometries (IV and V) and electronic structures. Both have practically all of the unpaired spin-density confined to nuclei located on the three-fold axis, in Co 3dz2, C 2s or H Is orbitals. Under certain conditions, a radical product of hydrogen-atom addition, H2Co(C0)3, was observed this species is believed to have a distorted trigonal bipyramidal structure in which the H-atoms occupy apical positions. 

These conclusions can be drawn by simple inspection of the molecular orbitals of PHS. The next question is the energetics of the interconversions of the various conformations. The result sketched in Fig. 27 refers to the Berry pseudorotation (J5) process of PHS. The square pyramid is a transition state for isomerization of the trigonal bipyramid, but the barrier is extremely small, in agreement with available experimental evidence. The more complex process ... 

Both Co2+ and Ni2+ also have the ability to make use of 4s, 4p, and 4d orbitals in another way by forming sp2d2 hybrids in the formation of complexes having a square-based pyramid structure. In fact, trigonal bipyramid and square-based pyramid structures are both observed for [Ni(CN)5J3. ... 

Let us first consider the case of a substitution reaction in a complex of a d6 ion such as Co3+ in a strong field. If the process takes place by an SN1 process, the five-bonded transition state may be presumed to have either a trigonal bipyramid or square-based pyramid structure. The orbital energies will be determined as follows ... 

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