Square Pyramids in The Solid State

By far the most common coordination geometry in the solid state is the octahedron followed perhaps by the tetrahedron. Trigonal bipyramids are quite rare, but there are a significant number of materials that are built from square pyramids. In this section, we shall look at three compounds with different electron counts. 

V2O5 is a solid-state compound with many catalytic and electronic uses including an electrode material for rechargeable lithium batteries. Its structure [60] is 

DOS for V2O5 when the distance between the vanadium atoms of one layer and the apical oxygen atoms in an adjacent one is 6.00 A in (a) and the experimental distance, 2.79 A, in (b). The projection of the apical oxygen lone pair, 2ai, in 17.62 is given by the shaded area. The vanadium projection is shown by the solid area. The Fermi level is indicated by f- The DOS plots are taken from Reference [61]. 

In 1988, Bednorzand Muller were awarded the Nobel Prize in Chemistry for the discovery of superconductivity in La2 xSrxCu04 with a T at 35 K [62]. We have discussed the electronic structure of this material in Section 16.5. Chu and coworkers prepared a related compound, YBa2Cu307, with a Tc = 92K [63]. The structure of this compound is shown in 17.63. The environment around Cu I is clearly a corner-shared square plane that runs along the x-axis to form a onedimensional chain. On either side of this chain in the z direction lies a puckered 

The density of states for the region around the Fermi level, 8f, in YBa2Cu307. The plots show the projection of copper and oxygen character by the dashed and dotted lines, respectively. The plots were adapted from Reference [64]. 

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The very labile dinitrogen complex [Mo(CO)(N2)(Ph2PCH2-CH2PPh2)2] has been described in detail. Its immediate precursor, [Mo(CO)(Ph2PCH2CH2PPh2)2], is obtained by the reaction of [Mo(N2)2-(Ph2PCH2CH2PPh2)2] with, for example, benzyl propionate under carefully controlled conditions (283, 322). This precursor is square pyramidal in the solid state, with carbon monoxide at the apex and an essentially vacant coordination position trans to it. The CO bond length is 1.192(12) A, and v(CO) = 1690 cm 1, and the nearest ap-... 

Mixed 6),Af-donor ligands such as Schiff bases are of interest in that they provide examples not only of square-planar coordination but also, in the solid state, examples of square-pyramidal coordination by dimerization (Fig. 28.6(b)). A similar situation occurs in the bis-dimethylglyoximato complex, which dimerizes by sharing oxygen atoms, though the 4 coplanar donor atoms are all nitrogen atoms. Copper(II) carboxylates are easily obtained by crystallization from aqueous solution or, in the case of the higher carboxylates, by precipitation with the appropriate acid from ethanolic solutions... 

A) between each unit, producing a tiara framework (Fig. 16). One Mo atom of the dinuclear core is in a square pyramidal environment and the other one, to which the pyridine ligand is bound, is in a distorted octahedral environment. The polyhedra share edges within the ring. In the solid state, the rings stack on top of each other, forming channels. 

Macrocycles attached to redox responsive groups such as ferrocene (78) can give selective transition metal ion receptors. The X-ray structure reveals a five-coordinate zinc with distorted square-pyramidal geometry bound to the four macrocycle nitrogens and an iodide. In the solid state the two ferrocenyl groups are positioned on the same side of the ligand with distances to the metal center of 5.347(7) and 6.120(8) A and these distances can be related to the redox behavior.689... 

These spectroscopic data support a square-pyramidal structure in solution with the phosphines mutually trans disposed, the chloride and the carbonyl group occupying the basal sites, and the hydride ligand located at the apex. This structure fully agrees with that found in the solid state, by X-ray diffraction analysis for the related compound OsHCl(CO)(PCy3)218, and for ab initio DFT (Becke 3LYP) methods for the model complex OsHC1(CO)(PH3)2.19... 

The analogous cationic pyridylbis(oxazoline)-copper complexes exhibit square pyramidal geometries in the solid state. As in the bis(oxazoline) series, the triflate is closer to the metal than the SbF6 counterion (2.36 and 2.49 A vs 2.90 A). A single molecule of water is bound to the copper center in the triflate complex 267b, whereas the SbF6 complex 268b accommodates two water molecules in the coordination sphere, Fig. 24 (197). 

As shown by ab initio studies with the anionic model species 49, the energy difference between the trigonal-bipyramidal (C2 symmetry) and the alternative square-pyramidal structure (C2v symmetry) is rather small (6.9 kJ mor1).40 From this result one may conclude that related zwitterionic A5S7-silicates, such as 47 and 48, should also have a chance to exist with a square-pyramidal /-coordination polyhedron in the solid state. The square-... 

CIS-VO2 complexes with tridentate Schiff-base ligands also appear, and they sometimes have five-coordinate square-pyramidal structures with one oxo-ligand at the apex. Many of these species have reversible features in their CVs at E = ca —0.07 V versus Ag/AgCl/MeOH and can undergo a photoreduction in the solid state which can be reversed by dissolution [110]. 

The tetranuclear sulfur and selenium derivatives [E(AuPPh3)4]2+ show a square pyramidal geometry with the chalcogen atom in the apical position [241]. These cations dimerize in the solid state through Au-E interactions (Figure 1.40). Further auration of sulfur and selenium to give the penta- and hexa-nudear species has been achieved and trigonal bipyramidal and octahedral structures have been proposed [242]. 

Complexes (66 R = H and n = 2) were five-coordinate square pyramidal monomers in py, six-coordinate species form.728 Compound (66 R1 = R2 = R = H), polymeric in the solid state (Figure 30), was considered to break down to five-coordinate monomers in CHCI3 and six-coordinate. in py. Changes in the spectrum of (66 R1 = R2 = R = H, n = 9) in CHC13 on dilution indicate that the complexes with n 4 are not monomers. Compounds (66 R = 3-MeO) were five-coordinate monomers except when R1 = R2 and n = 3.557 Compounds (66 R = 5-C1) tend to be polymeric solids. Complexes with (74) appear monomeric in the solid state except when n = 3. 

The Mo04+ core structure is found in MoVI chemistry in a relatively small number of authenticated structures and in a few suggested examples shown in Table 3. The simplest set of complexes which potentially contains the Mo04+ core is MoOX4(X = F, Cl, Br). While a square pyramidal structure seems probable for the chloro and bromo complexes,232 the fluoro complex is polymeric in the solid state and in solution as well.233-235... 

On account of their diamagnetism and spectral properties the [NiX2(dpe)2] complexes have a substantial square planar structure in the solid state, possibly with two weakly interacting anions in axial positions, as found in Nil2(diars)2 (vide infra). In solution either five-coordinate or square planar species are formed.1277 3283,1347 The ligand tep (II) easily forms diamagnetic five-coordinate complexes [NiX(tep)2]Y in the solid state and in solution as well.1276,1281,1282 The structure of [NiI(tep)2]I is essentially square pyramidal.1281... 

The synthesis of the X2M(Et2Dtc) complexes (X = Cl, Br, I M = As, Sb, Bi) is accomplished by the reaction of the M(Et2Dtc)3 complexes with MX3 in a 1 2 molar ratio in CHC13 (170). The structure of the Br2 As(Et2 Dtc) complex, which is monomeric and a nonelectrolyte in solution, in the solid state shows a loosely held centrosymmetric dimer with bridging bromide ions. The coordination geometry of the five-coordinate As(III) is intermediate between a square pyramid and a trigonal bipyramid, with a stereochemically active lone pair occupying a site of a distorted octahedral coordination sphere (Fig. 9). 

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