Seven-coordinate species

Iridium(V) complexes, although not common, take a variety of forms, including polynuclear and multidentate structures as well as five-, six- and seven-coordinate species, some of which are stable and have been characterized by X-ray crystallography. 

Various manganese complexes have also been investigated. In one study, six-coordinate compounds of type [MnXL]+ and seven-coordinate species of type [MnX2L] [where L = (293) and X is one of a range of... 

To finish this section, a particular type of Mo(VI) bisperoxo complex deserves consideration. Thiel and collaborators have prepared several seven coordinated species, 35, containing pyrazolylpyridine ligands bearing long alkyl side chains, which are therefore highly soluble in non-polar organic solvents. 

Mo(NO)(NCO)(S2CNEtj)2 2N2H4].113 114 These are also all reckoned to be seven-coordinate species, with one of two possible structures admitted as likely on the basis of NMR and IR data. The general relationships are indicated in Scheme 1. These compounds are of interest in the context of nitrogen fixation, and the observation that the azido ligand in... 

It is useful to classify the complexes by their coordination number. The majority of them are six-coordinate with octahedral structure, although four- and five-coordinate species are numerous, and even some seven-coordinate species are known. Table I shows a list of the main types of compounds reported in the literature. 

The proposed mechanism offac-mer isomerization and HMPA exchange in rLaCt HMPAE]. The central seven-coordinate species is either an interchange transition state, an intermediate or a stable molecule. 

A crystal structure of the 1 1 adduct of 1,10-phenanthroline and the seven coordinate species, [Mn(199)(H20)2](C104)2, indicates that the phenanthroline molecule is not coordinated to the manganese but is sandwiched between [Mn(I99)(H20)2]2+ units in a lamellar structure.136 The phenanthroline nitrogens are hydrogen bound to water molecules in adjacent complex ions the structure also appears to be held together by it donor/rc acceptor and dispersion forces. 

Support for the above conclusions was also obtained from an analysis of the Cd NMR and solid state MAS spectra of the five, six, and seven coordinate cadmium texaphyrin complexes 116, 156, and 157, respectively [70]. From the solid-state MAS results, a single tensor was observed for the five-coordinate complex 116 with a corresponding isotropic chemical shift of 194 ppm. The MAS spectrum of a complex prepared in the presence of pyridine, however, revealed two tensors, presumed to be due to a mixture of six and seven coordinate species. Based upon the isotropic chemical shifts and the symmetry of the tensors the six coordinate species was assigned to the isotropic shift at 188 ppm and the seven coordinate species assigned to the isotropic shift at 221 ppm. The MAS spectrum of the benzimidazole complex of cadmium texaphyrin consists of only a single... 

Seven-coordinate species MCl4(PMe3)3 have also been prepared.494,496,497 NbBr4(PMe2Ph)3 was prepared from reaction of Nb2Br10 with excess PMe2Ph in tol.502... 

The electrochemistry of a range of these seven-coordinate species was explored.649 Chemical reactions followed oxidation or reduction of most species. However, the more highly substituted complexes MX(CO)2(dppe)2 (X = H, Cl, Br, I) exhibited chemically reversible le oxidations in CH2C12/Bu4NPF6 and the 17e [MIIX(CO)2L2]+ cations were characterized by EPR spectroscopy (A(93Nb) 113 G for X = C1, H). 

Protonation of [M I(PF3)6] by cone. H2S04 yielded volatile, thermally unstable species [MiH(PF3)6].652,653 The formulation was consistent with H NMR and mass spectrometric studies for M = Nb. For M = Ta, H NMR detected a septet at 8, —8.7 ppm, consistent with coupling of six equivalent PF3 ligands to ligand H in a fluxional seven-coordinate species. 

The concepts rely also on our understanding of preferred coordination numbers and geometries. For example, for many metal ions six-coordination is common, but some stable examples of both five- and seven-coordination are known. Therefore, it isn t too much of a jump to consider a short-lived five- or seven-coordinate species existing for a species stable only in six-coordination - in other words, a transition state of lower or higher coordination number. As an example, six-coordinate octahedral is the overwhelmingly dominant coordination mode for stable cobalt(ni) complexes. Yet, in recent years, rare examples of isolable but usually very reactive compounds with five-coordination and seven-coordination have been prepared. It doesn t take too much of an act of faith to assert that such geometries form as short-lived transition state species in substitution reactions of this whole family of complexes. 

As a result of the process proposed, you could view the transition state as a five-coordinate species with two tightly bound outer-sphere exchanging ligands, or else as a seven-coordinate species with two very long bonds to the exchanging ligands. The descriptions amount to the same thing, in effect. 

As noted above, upon depressurization the amorphous A1P04 recrystallizes. This is the point at which the percentage of four-coordinated aluminiums jumps from 2% to 59%, while the percentage of five-coordinated aluminiums decreases from 48% to 38% and that of the six-coordinated aluminium atoms decreases sharply from 38% to just 3%. The seven-coordinated species disappear altogether upon recrystallization. As the crystal is depressurized... 

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