Square-based pyramidal species

In general, the R2BiX type compounds coordinate one two-electron donor (L) to afford a four-coordinate disphenoidal species R2BiX L, whilst the RBiX2 type compounds coordinate two two-electron-donors, giving a five-coordinate, square-based pyramidal species RBiX2-L2 (Figs. 2.2 and 2.3). 

Fig. 25.7 Octahedral substitution through (a) a square-based pyramidal species leads to retention of the initial cis-isomer but through (b), a trigonal bipyramidal species, may lead to isomerization.

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. 

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. 

Table 10 Selected Examples of Five-coordinated Species Showing Significant Distortions towards the Square-based Pyramidal Geometry...

A single crystal X-ray of the species derived from ethylene glycol, OsO(02C2H4)2, shows this to have a square-based pyramidal structure (Figure 29c).449-532... 

The condensation reactions of carbonyl metallates with neutral species, which are either coordinatively unsaturated or which will readily generate coordinatively unsaturated fragments, have also yielded a variety of mixed-metal clusters. The square-based pyramidal dianion [Fe5C(CO)i4] , for example, has been shown to react with a number of such species to yield octahedral FcjMC cluster compounds (Scheme 9) 269, 381). In some cases. 

If this is so, then the formation of [Fe5C(CO),5] could be compared with the generation of the bridged butterfly M5C(CO)isL (M = Ru, Os) species from the square-based pyramid M,C(CO), upon addition of L as discussed in Section III,E,2,a. 

Each XeFs group of the XeaFu species approximates to a square-based pyramid, with the xenon atom placed below the base. The F -Xe-Feq angles are 80° in both XeFs groups. On the other hand, each of the groups departs significantly from symmetry and the cis Feq-Xe- Fe,... 

SiFs as a molecule with its threefold axis tilted with respect to Cq or as a square based pyramid (an octahedral MFg less one F ligand) would not pack very differently from MFg species. The latter fit with least separation of the carbon-atom sheets when a threefold axis is perpendicular to the sheets. 

The general kinetic scheme shown in Scheme 8 has been developed for alkyl migration in [Mn(R)(CO)s] compounds. Thus, two pathways to the final product are avaUable (i) a second-order pathway (AiMn(R)(CO)5][L]) and (ii) a two-step sequence via the coordinatively unsaturated intermediate [Mn(COR)(CO)4]. In polar solvents, this intermediate may exist as a solvated hexacoordinate species, [Mn(COR)-(CO)4(S)] (S = molecule of solvent). In the absence of a suitably polar solvent, the intermediate is thought to adopt a square-based pyramidal conformation (99,100) with the acetyl ligand occupying a basal position, although a 7r-acyl derivative has also been proposed (101,102). 

In hydrated forms of Cu2+-doped H-, Na-, K- and Cs-rho a small part of the Cu + is octahedrally coordinated to six water molecules and undergoes free rotation at room temperature (6,7). This Cu + is located in the a-cages and has little interaction with the cage walls. The major part of the Cu + is coordinated to two water molecules and to oxygens in the zeolite lattice. This species is most likely either a square-planar complex located at site S3 or less likely a square-based pyramid located at site S2. An interesting feature of zeolite rho is that the nature of the hydrated species is independent of the nature of the cocation. This is quite different from the case in zeolite A where the location and hydration number of the major hydrated species is governed by the type of cocation. 

The cr-bonding framework in a square-pyramidal species may also be described in terms of an sp d hybridization scheme. The change in spatial disposition of the five hybrid orbitals from trigonal bipyramidal to square-based pyramidal is a consequence of the participation of a different d orbital. Hybridization of s,Px,Py,Pz and dyi yi atomic orbitals generates a set of five sj d hybrid orbitals (Figure 4.7b). 

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