Trigonal Bipyramidal and Octahedral Compounds

The positional isomers of trigonal bipyramidal and octahedral compounds can, in some cases, exist in enantiomorphic forms, and these are indicated in the two schematic Tables 13.5 and 13.6. Each isomer marked OA is devoid of an alternating axis of symmetry and can exist in an enantiomorphic mirror image form (not shown). 

It should be noted from Table 13.5 that with five different ligand groups, 10 different positional isomers are possible. Each of these is a diastereoisomer and can exist in a pair of enantiomorphic forms, making a total of 20 possible isomers. 

Octahedral isomers of types PX4Y2 and PX3Y3 have been characterised by NMR spectroscopy, 

Some isomers have been shown to exist in equilibrium. 

The possible alternative trigonal bipyramidal isomers based on single ligand groups (Table 13.5) are generally more difficult to isolate than tetrahedral isomers. This is probably because of their ready interconversion by pseudorotation in many cases, or because the non-equivalence of equatorial and axial positions favours the adoption of only one (the most stable) isomeric form. 

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Conformational equilibria of trigonal bipyramid and octahedral compounds... 

Molecules satisfying the requirements for optical activity are also known as chiral molecules. Chirality arises in pyramidal and tetrahedral phosphorus compounds when the substituent groups are all different (Chapter 3.2). It may also arise in trigonal bipyramidal and octahedral compounds when at least three different kinds of substituent molecules are present. 

A range of complexes with the novel quadridentate and tridentate ligands (53 L = P or As, or mixed P, As) and (54 L = P or As) have been prepared and characterized by lH n.m.r. and u.v.-visible spectroscopy,232,233 Trigonal-bipyramidal or octahedral compounds were generaly obtained, for example, the species [IrX (o-... 

In virtually all its stable compounds carbon forms four bonds and has coordination numbers of 2 (=C— or =C=), 3 (=CQ, or 4, with linear, triangular (planar), and tetrahedral geometries, respectively CO has coordination number 1. In interstitial carbides (Section 7-3), certain metal cluster compounds1 (Section 7-9), and very stable trigonal bipyramidal and octahedral penta- and hexa(aurio)methanium cations of the type (LAu)5C+ and (LAu)6C2+, where L is a phosphine,2 carbon atoms are found with coordination numbers of 4, 5, or 6. Coordination number 5 is also found in compounds with bridging alkyls such as Al2Me6, in some carboranes (Section 5-12), and in reactive carbocations.3... 

Organoruthenium compounds form tetrahedral, trigonal bipyramidal and octahedral structure of four, five and six-coordination, respectively. An example of a tetrahedral structure is (l,3,5-C6H2(Me)3)4Ru [36], those of trigonal bipyramidal are (o-toIyl)RuCl(CO)(PPh3)2 [59], Ru(CO)4(P(OMe)3) [60] and... 

The most common structures of arsenic compounds are tetrahedral and pyramidal, which are similar when the sterically active lone pair is counted. Tetrahedral symmetry holds the potential for chirality and indeed many chiral organoarsenic compounds have been prepared. Arsenic may also use d orbitals for (d-d)n bonding and for hybridization with s2 and p3 orbitals, resulting in trigonal bipyramidal or octahedral structures. In the former the more electronegative substituents occupy the apical position. 

Low-oxidation state iron compounds are stabilized by electron acceptor ligands, especially carbon monoxide, isocyanide, and phosphorus donors. The most common coordination geometry encountered in iron compounds is octahedral, but tetrahedral, trigonal bipyramidal, and square pyramidal are also represented. There are only a few compounds with iron in the -II state, although reduction of [Fe(CO)s] with sodium amalgam in THF... 

At the platinum centre, the Pt(0) compounds are 3-coordinate and planar, or 4-coordinate and tetrahedral, the Pt(II) compounds are 4-coordinate and planar, or 5-coordinate and trigonal bipyramidal, and the Pt(IV) compounds are 6-coordinate and octahedral. Often, one of the ligands on the platinum is a phosphine, and 31P NMR spectroscopy has played an important part in structural determinations, though many structures have now been established by X-ray diffraction. 

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