Coordination states

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Stabilisation of unusual oxidation and coordination states by the ligands OSF5, OSeF5 and OTeF5. K. Seppelt, Angew. Chem., Int. Ed. Engl., 1982, 21, 877-888 (138). 

Derived from the German word meaning devil s copper, nickel is found predominantly in two isotopic forms, Ni (68% natural abundance) and Ni (26%). Ni exists in four oxidation states, 0, I, II, III, and IV. Ni(II), which is the most common oxidation state, has an ionic radius of —65 pm in the four-coordinate state and —80 pm in the octahedral low-spin state. The Ni(II) aqua cation exhibits a pAa of 9.9. It forms tight complexes with histidine (log Af = 15.9) and, among the first-row transition metals, is second only to Cu(II) in its ability to complex with acidic amino acids (log K( = 6-7 (7). Although Ni(II) is most common, the paramagnetic Ni(I) and Ni(III) states are also attainable. Ni(I), a (P metal, can exist only in the S = state, whereas Ni(lll), a cT ion, can be either S = or S =. ... 

Electrochemical studies performed in the 7 x Cys-Aspl4 D. afri-canus Fdlll indicate that the reduced [3Fe-4S] center can react rapidly with Fe to form a [4Fe-4S] core that must include noncysteinyl coordination (101). The carboxylate side chain of Asp 14 was proposed as the most likely candidate, since this amino acid occupies the cysteine position in the typical sequence of a 8Fe protein as indicated before. The novel [4Fe-4S] cluster with mixed S and O coordination has a midpoint redox potential of 400 mV (88). This novel coordinated state with an oxygen coordination to the iron-sulfur core is a plausible model for a [4Fe-4S] core showing unusual spin states present in complex proteins (113, 114). 

Almost any class of reduced nitrogen compound can serve as the nitrosatable precursor of an N-nitroso compound. To make matters worse, every nitrogen coordination state from primary to quaternary has been converted to a nitrosamine. A glance at the list of some of the known nitrosatable substrates given in Table II illustrates what a wide variety of N-nitroso compounds analysts might expect to encounter in our complex environment. 

In these glasses (Table 5.6) the coordination state of aluminium depends on its chemical environment and can only be entirely fourfold when the Ca/Al... 

Zn -PDF, 37 pM versus E. coli Fe -PDF), it was successfully used to provide co-crystals bound in the active site of both Co - and Zn -E. coli PDF [58], These structures reveal that the H-phosphonate binds to the metal in a monodentate fashion, adopting a tetrahedral coordination state similar to that of the native resting state of the enzyme. This is in contrast to later co-crystal structures obtained with more potent hydroxamic acid or reverse hydroxamate inhibitors, which bind to the metal in a bidentate fashion vide infra). Presumably these bidentate inhibitors mimic the true transition state of the enzyme, in which the metal centre slips to a penta-coordinate geometry in order to activate the Wformyl carbonyl of the substrate [56, 67]. 

Analysis of the 27A1 NMR spectra has confirmed the structure of the aluminium di-Schiff bases complexes being derivatives of trans-1,2-dia-minocyclohexane. The A1 chemical shift values of 6.57 and 39.48 ppm indicated the presence of dynamic equilibrium between five- and six-coordinated states in CDC13 solution. 

P nuclear magnetic resonance (NMR) spectroscopy has been of great use in determining the coordination state and stereochemistry of the phosphorus atom at the spiro position in spirophosphonia compounds, spirophosphoranes and spiroperphosporanides. The 31P chemical shift is also sensitive to the nature of the atoms directly bonded to the spiro phosphorus center and the size of rings of the spirocyclic system. 

Acyclic boryloxyalkylphosphines with tricoordinated phosphorus and boron are capable of forming cyclic betaine structures with four-coordinated P and B atoms. The ability to be converted into a more stable four-coordinated state accounts for many chemical transformations of boryloxyalkylphosphines. Diphenylboryloxymethyl(methyl) phenylphosphine (92) readily disproportionates to give 1,3,2,5-dioxabora-taphosphoniarinane (103). [Eq. (60)] (83IZV2541). Similar interaction is... 

Betaines may be considered to be the intermediate products in the displacement of the C—O fragment from the P—C—O—B system, whereas the addition of aldehydes to the P—C—O—B system constitutes the first stage. This reaction is due to the fact that phosphorus and boron atoms can change their coordination reversibly and convert into the tetra-coordinated state. The displacement of one aldehyde by another is carried out in a solvent or in excess aldehyde. In general this reaction is represented by the following scheme [Eq. (107)]. 

Fig. 24 Calculated structures of cationic phenoxy-amine complexes in the absence of 1-hexene (a), in the presence of 1-hexene (1-hexene-coordinated state) (b), and for transition state (c) (polymer chain model methyl group). Reproduced with permission from Saito et al. [27]. Copyright 2006, American Chemical Society...

The coordination numbers of the Ln3+ ions in water are now well established from different experimental techniques (214-221). The lighter La3+-Nd3+ ions are predominantly nine-coordinate, Pm3+ Eu3+ exist in equilibria between nine- and eight-coordinate states and the heavier Gd3+-Lu3+ are predominantly eight-coordinate. The change in coordination number is also reflected in the absolute partial molar volumes, U°bs, of several Ln3+ ions determined in aqueous solutions (222,223). 

A mechanism (from comparison with —12.1 cm3 mol-1 for [Ti(H20)6]3+, Section III.C). The EXAFS study showed that an equilibrium between seven- and eight-coordinate states is possible which would explain the very low energy barrier between the transition state (eight-coordinate for an associative mechanism) and the ground state [Eu(H20)7]2+ (228). 

Available textbooks and reviews provide a detailed introduction to MB spectroscopy (e.g., Hawthorne 1988 Burns 1993). MB spectra provide quantitative information about the distribution of Fe among its oxidation and coordination states (e.g., octahedrally coordinated Fe3+), identification of Fe-bearing phases, relative distribution of Fe among those phases, and can help to constrain crystallinity and particle size. 

Step 1. The substrate, RH, associates with the active site of the enzyme and perturbs the spin-state equilibrium. Water is ejected from the active site and the electronic configuration shifts to favor the high-spin form in which pentaco-ordinated heme Fe3+ becomes the dominant form-binding substrate. In this coordination state, Fe3+ is puckered out and above the plane in the direction of the sixth ligand site. The change in spin state alters the redox potential of the system so that the substrate-bound enzyme is now more easily reduced. 

Dmb ligand is replaced by His to generate the His-on conformation (Figure 15.8B). This His residue is part of a catalytic triad which controls the coordination state of cobalt (His-on/ His-off) by modulating the protonation state of the histidine. 

In the dithionite-reduced state (Figure 17.11a), PN, the P-cluster can be considered as two [4Fe-3S] clusters bridged by a hexa-coordinate sulfur. In the Pox state, which is oxidized by two electrons relative to PN, two of the iron atoms Fe5 and Fe6 have moved away from the central sulfur atom, and are now coordinated by the amide nitrogen of Cysot87 and the hydroxyl of Seral86, maintaining the irons in a four-coordinate state. 

The transformation of RhCl(PH3)2(HC=CH) to RhCl(PH3)2(C=CH2) has been calculated (MP2) to be exothermic by 7.8 kcal.mol"1. The intraligand 1,2-hydrogen shift mechanism found in the Ru11 system is not relevant to the present rhodium case. Starting from a T 2 C=C complex, both systems give a metal-( T]2 C-H) species in a subsequent step. In the case of the d6 Ru system this ri2 C-H complex is an intermediate. In contrast, the T 2 C-H coordinated state is a transition state in the d8 Rh1 system, the oxidative addition being a very facile process. 

In the trivalent state the stereochemistry of the actinides is similar to the lanthanides, that is, eight coordinate. However, higher coordination states are known for some trivalent actinides, for instance, UC13 exists in the nine coordinate state. In the tetravalent state the species normally encountered are eight coordinate. 

As shown in Fig. 8-34, when the most probable electron level of the reductant particle is higher in the ligand-coordinated state cred(chydrated state ereix i). ibe transfer of anodic electrons occurs at higher energy levels (at less anodic potentials) with the ligand-coordinated reductant particle than with the simply hydrated reductant particle. In such a case the complexation of redox particles will accelerate the anodic transfer of redox electrons. 

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

Region III. If relaxation is controlled by the difference in precessional frequency between the free and coordinated states, that is, if... 

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

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