Structures, square-based pyramidal

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 ... 

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. 

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

It is made by reaction of RuClj with PPh in methanol, and forms shiny black crystals [881, 882], The X-ray crystal structure shows the molecule to have a distorted square-based pyramidal structure, a phosphorus atom forming the axial bond (Ru-P 2.230(8) A) while the basal plane has two trans chloro ligands (Ru-C1 2.387(7) and 2.388(7) A) and two trans phosphine ligands (Ru-P 2.374(4) and 2.412(6) A). It can be regarded as octahedral, with the sixth position blocked by a phenyl ring (Fig. 1.39) [883]. The catalytic efficacy of the complex may well depend on the availability of this vacant coordination site. 

Finally, a square-based pyramid structure is reached (the transition state for pseudorotation) in which the two ax/a/ fluorines and two of the equatoriar fluorines are equivalent. Motion continues until the two equatorial fluorines occupy axial positions and the two axial fluorines occupy equatorial positions (the remaining equatorial fluorine does not move). 

Hartree-Fock 6-3IG calculations show that pseudorotation in phosphorous pentafluoride is a very low energy process (6 kcal/mol) and that the square-based pyramid structure is indeed a transition state. 

Tetraethylammonium pentachloroindate(lll) is a white crystalline solid, mp 285° (dec.), slightly soluble in ethanol at 20° (more so at the boiling point) and also soluble in acetone and dichloromethane. The IR7,8 spectrum shows absorptions at 294 (s), 282 (s), 268 (s), 152 (sh), and 142 cm 1 Raman emissions have been reported9,10 at 294 (s), 287 (sh), 194 (w), 167 (m), 123 (m), and 106 (m) cm"1. The crystal structure determination11,12 shows that the anion is essentially square-based pyramidal, an unusual stereochemistry for main group elements MX5 species. 

---