Square-based bipyramid complex

Although the majority of complexes have structures that are linear, tetrahedral, square planar, or octahedral, a few compounds have a trigonal bipyramid structure. Most notable of these are Fe(CO)5, Ni(CN)5ji, and [Co(CN)5]3T Some complexes having a coordination number of 5 have the square-base pyramid structure, including [Ni(CN)5 3. Although it is not particularly common, the coordination number 8 is found in the complex [Mn(CN)8]4-, which has a cubic structure with CIST ions on the corners. 

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 ... 

Starting Oh complex Trigonal bipyramid Square-based planar... 

In general, it is not possible to predict the stereochemistry about the separate copper(II) ions in most cases they are the same and may or may not be related by a centre of symmetry. The actual stereochemistries produced are recognizably the same as those occurring in mononuclear copper(II) complexes (Figure 19.1). The most common is that of square-based pyramidal with rhombic coplanar, compressed tetrahedral, trigonal bipyramidal and elongated rhombic octahedral stereochemistries all occurring. 

E7.5 With five-coordinate complexes, the two possible geometries are trigonal bipyramidal and square-based... 

The amended VSEPR model predicts two forms of five-coordination, and experimental chemistry has clearly identified many examples of both forms. These limiting structures are square-based pyramidal (or, simply, square pyramidal) and trigonal bipyramidal (Figure 4.13). The classical square-based pyramidal shape is formed simply by cleaving off one bond from an octahedral shape, which leaves the metal in the same plane as the four square-based ligands. In reality, almost no complexes exhibit this shape, but rather adopt a distorted... 

The limiting structures for 5-coordination are the trigonal bipyramid and square-based pyramid. In practice, many structures lie between these two extremes, and we have already emphasized that the energy differences between trigonal bipyramidal and square-based pyramidal structures are often small (see Section 2.11). Among simple 5-coordi-nate complexes are trigonal bipyramidal [CdCls], ... 

For some Ni(II) complexes, there is only a small energy difference between structure types. In Section 19.7, we stated that both trigonal bipyramidal and square-based pyramidal [Ni(CN)5] ions (equation 21.90) are present in crystals of [Cr(en)3][Ni(CN)5]1.5H20. In the anhydrous salt, however, the anions are square-based pyramidal. It is impossible to give a simple interpretation of these observations which may be attributed to a subtle balance of steric and electronic effects . 

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