Five-Coordinated Al

Al occurs in well-defined pentacoordinated sites in a number of crystalline materials, including andalusite (Alemany and Kirker 1986, Lippmaa et al. 1986, Dec et al. 1991), 

augelite, after Bleam et al. (1989), D. grandidierite, from Smith and Steuemagel (1992). 

Jansen et al. (1998) have demonstrated an approximately linear relationship between the isotropic chemical shifts of a number of these crystalline Al compounds and a parameter R describing the angular distortion of the Al polyhedron from an ideal trigonal bipyramid configuration, defined as 

These results suggest that distorted trigonal bipyramidal configurations are most prevalent in these compounds, with the exception of a complex hydroxy-aluminosilicate, vesuvianite (Phillips et al. 1987), in which the Al site is closer to a square pyramidal configuration. 

In addition to the crystallographically well-defined compounds, there are a number of other compounds which contain a broad Al resonance at about the position of the Siso values for Al (25-40 ppm). The compounds containing this resonance are often 

---

MQMAS clearly shows that the middle region in the 1 D MAS spectmm is due to combination of five coordinate Al and distorted four coordinate aluminums. 

Thermal desorption spectra of CO2 from a titania surface are shown in figure 2. It revealed two desorption peaks at temperature ca. 175 and 200 K. As reported, surface of titania have two structures which is similar to the results fomd by Tracy et al. [7]. Based on their study, it was confirmed that one peak at ca. 170 K was attributed to CO2 molecules bound to regular five-coordinate Ti site considered as the perfected titania structure. The second peak at ca. 200 K considered as the CO2 molecules bound to Ti referred to the... 

Figure 18.4 Structures of heme/Cu oxidases at different levels of detail, (a) Position of the redox-active cofactors relative to the membrane of CcO (left, only two obligatory subunits are shown) and quinol oxidase (right), (b) Electron transfer paths in mammalian CcO. Note that the imidazoles that ligate six-coordinate heme a and the five-coordinate heme are linked by a single amino acid, which can serve as a wire for electron transfer from ferroheme a to ferriheme as. (c) The O2 reduction site of mammalian CcO the numbering of the residues corresponds to that in the crystal structure of bovine heart CcO. The subscript 3 in heme as and heme 03 signifies the heme that binds O2. The structures were generated using coordinates deposited in the Protein Data Bank, lari [Ostermeier et al., 1997] Ifft [Abramson et al., 2000] (a) and locc [Tsukihara et al., 1996] (b, c).

In another publication, Holsboer et al. [280] have investigated bond properties in the five-coordinated Ir(I) complexes (PPh3)3lr(CO)H, (PPh3)3lr (CO)CN, and the tetracyanoethylene, fumaronitrile, and acrylonitrile adducts of (PPh3)2lr(CO)Cl by X-ray photoelectron and fr Mossbauer spectroscopy. 

Of crucial significance in deciding between various models have been estimates of the number of copper atoms required to transform the surface into a (2 x 3)N phase. This was the approach adopted by Takehiro et al 2 in their study of NO dissociation at Cu(110). They concluded that by determining the stoichiometry of the (2 x 3)N phase that there is good evidence for a pseudo-(100) model, where a Cu(ll0) row penetrates into the surface layer per three [ll0]Cu surface rows. It is the formation of the five-coordinated N atoms that drives the reconstruction. The authors are of the view that their observations are inconsistent with the added-row model. The structure of the (2 x 3)N phase produced by implantation of nitrogen atoms appears to be identical with that formed by the dissociative chemisorption of nitric oxide. 

Markies et al. have studied the coordination of zinc to cyclic and acyclic polyether ligands. In some cases the zinc is not coordinated within the ligand cavity but only through two of the ether donors in the macrocyclic ring.360 The bis(methoxyethyl)ether complex of zinc diphenyl (41) is five-coordinate with three ether oxygens coordinating from the acyclic ligand.361... 

Casey et al. have studied the decarbonylation reactions of [cis-(OC)4M(MeCO)(PhCO)], in which M is Mn or Re (16,17). These complexes lose a carbonyl ligand to form five-coordinate intermediates of the type [(OC)3M(MeCO)(PhCO)]. Reversible methyl migration proceeds much more rapidly than does phenyl migration. In the course of these studies, a phosphine substituted rhena-/3-diketonate complex, [fac-(OC)3(Et3P)Re(MeCO) (PhCO)], was prepared. 

Downs et al. reported the synthesis of methylzinc tetrahydridoborate 164 by the two routes shown in Scheme 99.230 The compound is an extremely moisture and oxygen sensitive, colorless, polycrystalline solid, whose solid-state structure was determined by X-ray analysis. Figure 76 shows that 164 consists of helical polymers of alternating methylzinc and tetrahydridoborate ions. The zinc atom is formally pentacoordinate, making this one of the few organozinc compounds with five-coordinate zinc atom. 

---