Coordinate position

Perhaps the most important complex of iron(II) is heme (or haeme). Haemoglobin, the iron-containing constituent of the blood, consists essentially of a protein, globin, attached through a nitrogen atom at one coordination position of an octahedral complex of iron(II). Of the other five coordination positions, four (in a plane) are occupied by nitrogen atoms, each of which is part of an organic... 

The other two coordination positions are available for groups of the fiber having free electron pairs, as ia (51) where B is —NH2, —NH—, or —OH. 

Figure 16.23 Overview of the structure of the SV40 virus particle, showing the packing of pentamers. The subunits of pentamers on fivefold positions are shown in white those of pentamers in six-coordinated positions are shown in colors. The six colors indicate six quite different environments for the subunit. (Courtesy of S. Harrison.)...

When the two coordinating centres are not in separate ions but are joined by bridging groups, the isomers are often distinguished as coordination position isomers as is the case for ... 

Fumes and vapors discharged to the environment via a chimney form a plume, which is approximately cone shaped. Mathematical modeling of dispersal rates is possible. The Gaussian dispersion model is commonly used to calculate the concentration of pollutants at coordinate positions X, Y and Z. (The coordinates are measured from the plume centerline.) The equation used is ... 

The sixth coordination position of Fe2+ in heme is occupied by a water molecule that can be replaced reversibly by oxygen to give a derivative known as oxyhemoglobin, which has the bright red color characteristic of arterial blood. 

The coordinate positions for equivalent atoms provided by F 5 are1)... 

First, we must say a word about the naming of A. For many years these species were called carbonium ions , though it was suggested as long ago as 1902 that this was inappropriate because -onium usually refers to a covalency higher than that of the neutral atom. Nevertheless, the name carbonium ion was well established and created few problems until some years ago, when George Olah and his co-workers found evidence for another type of intermediate in which there is a positive charge at a carbon atom, but in which the formal covalency of the carbon atom is 5 rather than 3. The simplest example is the methanonium ion CHj (see p. 770). Olah proposed that the name carbonium ion be henceforth reserved for penta-coordinated positive... 

Preparations from Other Organocobalt III) Compounds a. Addition, Exchange, or Removal of an Axial Ligand. This reaction is frequently used to prepare organocobalt(III) complexes with different ligands (usually neutral bases) in the sixth coordination position. The... 

A) Reactions involving only one coordination position (the Co—C bond), where the only role of the surrounding solvent or medium is to facilitate the gain or loss of a proton by the organo-ligand. 

C) Reactions involving ligands in two adjacent (i.e., cis) coordination positions. They include certain insertion reactions (e.g., the insertion of cyanide into the Co—C bond) and the reverse elimination reactions. 

Reactions of group C, which involve interaction between two unidentate ligands in adjacent coordination positions, have so far only been reported for the pentacyanides, most of which undergo reactions to the type ... 

The iron of unoxygenated myoglobin lies 0.03 nm (0.3 A) outside the plane of the heme ring, toward His F8. The heme therefore puckers slightly. When O2 occupies the sixth coordination position, the iron moves to within 0.01 nm (0.1 A) of the plane of the heme ring. Oxygenation of myoglobin thus is accompanied by motion of the iron, of His F8, and of residues linked to His F8. 

Figure 6-1. Heme. The pyrrole rings and methylene bridge carbons are coplanar, and the iron atom (F62 ) resides in almost the same plane. The fifth and sixth coordination positions of Fej are directed perpendicular to—and directly above and below—the plane of the heme ring. Observe the nature of the substituent groups on the (3 carbons of the pyrrole rings, the central iron atom, and the location of the polar side of the heme ring (at about 7 o clock) that faces the surface of the myoglobin molecule.

As we turn to complexes with only four groups bound in a planar arrangement and two potential coordination positions open it should not be too surprising... 

One study has been made with a chelate complex, tran5-l,2-diaminocyclohexane-tetraaceatomanganate(III), which is either pentadentate or hexadentate, a water molecule occupying the sixth or seventh coordination position respectively, and hence chelation of the oxalate is very unlikely. The reaction is first-order both in oxidant and oxalate and is retarded by increase of acidity. The HC204 ion is, therefore, attacked more slowly than the 204 " ion but both forms are oxidised as follows... 

Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). 

One obvious way in which to attach a nitroxide group to B12 is to simple alkylate Cob(I)aiamin with a suitable nitroxide derivative. This would result in having the nitroxide covalently bound to the corrinoid at the upper axial coordination position of the cobalt. Such a procedure is outlined in Fig. 19. In this reaction 4-bromoacetamido 2,2,6,6-tetra-methylpiperidine-N-oxyl is used to alkylate Cob(I)alamin. This results in a Co(III)-nitroxalkylcobalamin. The corresponding cobinamide can then be produced by hydrolyzing the ribose-phosphate linkage (119). 

Another chemically more interesting spin labeled B12 derivative involves coordinate attachment of the nitroxyl function to the cobalt atom of a cobinamide. Fig. 22 shows a reaction in which an alkyl cobin-amide is mixed with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl. The nitroxide displaces water from the 6th coordination position very slowly and therefore this reaction is usually allowed to proceed for a few days with a large excess of nitroxide. From the properties of the coordinated nitroxide derivative discussed below, it is certain that the cobalt is coordinated by the N—O functional group. An analogous compound to that shown in Fig. 22 can be made with a similar nitroxide in which the 4-hydroxyl-group is missing. In this case the N—O-function is the only basic site on the molecule and therefore must be the position of attachment to the cobalt 119). 

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