Square-based pyramidal/trigonal

Os(PPh3)3X2 Os(CO)s 573 Square-based pyramidal Trigonal bipyramidal... 

R. S. Berry. Known as the Berry pseudorotation, the mechanism involves the trigonal bipyramid (D)fl) passing through a square based pyramid (C4v) as shown in Figure 14.8. This behavior is somewhat similar to the inversion of the ammonia molecule (C3 ) as it passes through a planar (D)fl) structure. 

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 ... 

Having rationalized that the transition state should be a square-based pyramid, it should be mentioned that there are numerous cases in which the transition state appears to be a trigonal bipyramid. We know that because the substitution occurs with a change in configuration. From the foregoing discussion, we would expect this to occur with first-row transition metals because if 11.48 Dq must be sacrificed, this would be more likely if Dq is smaller (which it is the case for first-row metals). If a trigonal bipyramid transition state forms, there would be more than one product possible. This can be... 

If this reaction were to take place by formation of a square-based pyramid transition state, the product would have a trans configuration. However, if the transition state is a trigonal bipyramid, the incoming ligand, Y, could enter either cis or trans to A. 

For five-metal clusters, two "parent polyhedra may be considered, viz., the trigonal bipyramid and square-based pyramid. 

Trigonal bipyramid Square-based pyramid Ta(NEt2)5l Nb(NMe2)5], [NbOCl4] , [NbSCl3(Ph3PS)]... 

ERO — elongated rhombic octahedral TB = trigonal bipyramidal SBP = square-based pyramidal L = linear Td = tetrahedral Tr = trigonal. ... 

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. 

For a discussion of oxyanion coordination numbers see Chapter 15.5, structures I-IV. b tsglyo — JV-tosylglycinate. cCTd = compressed tetrahedral SBP - square-based pyramidal ERO = elongated rhombic octahedral TB = trigonal bipyramidal LB = long bipyramidal SP - square planar. 

Compiled from Table 9.2 ref. 1065. bSP = square planar CTd = compressed tetrahedral SBP = square-based pyramidal TP = trigonal pyramidal ETO = elongated tetragonal octahedral. 

ETO = elongated tetragonal octahedral SBP = square-based pyramidal TB = trigonal bipyramidal ERO = elongated rhombic octahedral CRO = composed rhombic octahedral SP - square planar O = octahedral CTO = compressed tetragonal octahedral 7C = seven coordinate cis-O = cis-octahedral RC = rhombic coptanar TO = tetragonal octahedral CTd — compressed tetrahedral. 

The structural studies of nitrosyls have shown to date that the bent nitrosyl ligand invariably occurs at the apical position of a square based pyramid or a distorted octahedron in which the metal ion configuration assuming NO- coordination is d6. Despite numerous electronic structural descriptions (168, 169,198-200), it is not totally clear why fully bent nitrosyls with M—N—O bond angles of 120° have not been found in other geometries such as the square plane and the trigonal bipyramid. 

Certain discrete monomeric compounds of this class are known for M = Nb and Ta, and for M = Nb both the dimethylamide and piperidide have a central square-based pyramidal MN5 unit distorted toward a trigonal bipyramid (Figure 3).150 However, the related Ta(NEt2)5 molecule has a TaN5 unit more closely approximating the idealized Dih geometry.151... 

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