Square-based pyramid

Indium clusters have also recently been characterized, notably in intermetallic compounds. Thus, the Zintl phase, Rbzinj, (prepared by direct reaction between the two metals at I530°C) has layers of octahedral closo-lnf, clusters joined into sheets through exo bonds at four coplanar vertices. These four In atoms are therefore each bonded to five neighbouring In atoms at the comers of a square-based pyramid, whereas the remaining two (Irans) In atoms in the Ine cluster... 

Slniclure of tetragonal SnO (and PbO) showing (a) a single square-based pyramid ( Sn04 arrangement of the pyramids in layers, and (c) a plane view of a single layer. 

The Vickers hardness measurement uses a square based pyramid of about 100 pm height as the indenter. The included angles between opposite faces are a = 136°. This corresponds to the tangential angle of an ideal ball impression, considered to have a diameter equal to 0.375 times that of the ball1 . The hardness value is equal to the applied force P in newtons divided by the actual area of impression in mm2. 

Zinc complexes of cyclohexane-1,2-dione bis(thiosemicarbazone) have been formed in the neutral and doubly deprotonated form. The X-ray structures show distorted square-based pyramidal geometries for both complexes [ZnL(OH2)] and [Zn(H2L)Cl]Cl with the water or chloride occupying the axial position.886 The binuclear complex bis[l-phenylglyoxal bis(3-piperidylthiose-micarbazone)zinc] was synthesized by electrochemical methods and reveals a Zn2L2 with two tetradentate bis-thiosemicarbazone ligands bridging the metal centers.887... 

We can see so many lanthanide clusters behave as efficient SMMs in those p -O-fixed complexes. Especially, the energy barrier record (530 K) for polynuclear lanthanide SMMs, held by the p5-0-fixed Dy5 with a square-based pyramid, has not been broken by the end of 2013. Such a structural characteristic should support the isolation of a robust SMM to a great extent, which provides a change to explore new methods for achieving SMMs with high barriers. 

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. 

Geometrical isomers are possible for complexes having a square-based pyramid structure. For example, the structures for a complex having the structure MLX2Y2 show that cis and trans arrangements are possible for the ligands in the base. 

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. ... 

Assume that the complex MLX2Y2 has a square-base pyramid structure with all ligands able to bond in all positions in the coordination sphere. How many isomers are possible ... 

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 ... 

Thus, the fact there is no isomerization during substitution is consistent with the transition state being a square based pyramid for second and third row metals. As shown in Figure 20.4, substitution would give a product having the same configuration as the starting complex. 

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. 

A catalytically inactive, (tyrosinato)Cu(II)-containing form of GO (pH 4.5 acetate buffer) has been characterized by X-ray crystallography (119, 120) at 1.7-A resolution. Fig. 6 shows the active site. The Cu(II) ion is in a distorted square-based pyramidal environment of a tyrosine (Tyr 495) ligand in the apical position where it is probably bound in its protonated phenol form, a second (modified) tyrosinate (Tyr 272) as well as two histidines (His 496, His 581) in equatorial positions. The fifth coordination site is occupied by a buffer derived acetate that in the active en-... 

In 1996, two reports appeared (202, 211) where it was shown for the first time that a (phenolato)copper(II) complex can be electrochemically or chemically reversibly converted to a relatively stable (phenoxyl)copper(II) species in CH2C12 or MeCN solution. Halfen et al. (202) reported the synthesis and characterization of four square-base pyramidal (phenolato)copper(II) precursors using the pendent... 

Tolman (144, 202) and Wieghardt (203, 204) and their co-workers used amine macrocycles with a 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms (Table VIII). In all cases, square-base pyramidal (phenolate)copper(II) precursor complexes were isolated and in many instances structurally characterized by X-ray crystallography. Depending on the number of coordinated phenolates, these complexes are reversibly one-electron oxidized yielding the (phenoxyl)copper(II) species that were characterized in solution by UV-vis, EPR, and RR spectroscopy. 

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

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