Metal—ligand bonds structures

As briefly stated in the introduction, we may consider one-dimensional cross sections through the zero-order potential energy surfaces for the two spin states, cf. Fig. 9, in order to illustrate the spin interconversion process and the accompanying modification of molecular structure. The potential energy of the complex in the particular spin state is thus plotted as a function of the vibrational coordinate that is most active in the process, i.e., the metal-ligand bond distance, R. These potential curves may be taken to represent a suitable cross section of the metal 3N-6 dimensional potential energy hypersurface of the molecule. Each potential curve has a minimum corresponding to the stable... 

X-ray structural studies of the diamagnetic anion (406) confirm that the Ir(-I) center is in a distorted coordination geometry intermediate between square planar and tetrahedral, with the P donor atoms in a cis position. The metal-ligand bond distances do not show significant changes among (404), (405), and (406). The Ir1/0 and Ir0/(-1) redox couples are measured at easily accessible potentials and are solvent dependent. 

All borabenzene-metal complexes investigated structurally so far show very similar patterns for the ligand geometry (Table I) and for the metal-ligand bonding (Table II) only the cobalt complex 6 deserves separate consideration (see below). 

Among the relevant points to emerge from these studies is that yes, the structural differences between the oxidized and reduced forms of the blue copper proteins are rather small, but the structures are also quite similar to those of the apoproteins. Thus, the presence of the copper ion, in either oxidation state, does not impose any significant strain on the peptide chain. The amount of strain in the metal-ligand bonds has been debated, but it is clearly not a huge quantity in view of the fact that the affinities of the protein for the metal ions are quite high. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

In a metalloid cluster [2] more metal-metal bonds than metal-ligand bonds are involved, which means n > r. The largest structurally characterized compounds of this type contain 77 A1 or 84 Ga atoms, respectively [3, 4], Metal-metal bonds dominate these clusters and the framework of the resulting metal-metal bonds exhibits a geometry similar to the bulk metal itself. With respect to the Greek word ei8o< (idea, prototype) the suffix -oid indicates that the bulk metal element is actually visible in the metal atom core of the metalloid or more generally, elementoid clusters. 

As seen, the only significant structural variations are concerned with the metal-ligand bonds, thus supporting that, in this case, the first reduction is metal-centred. 

The crystal structure of poplar apoplastocyanin has been reported at 1.8 A resolution [42]. The structure closely resembles that of the holoprotein, and positions of the Cu-binding residues are different by only 0.1-0.3 A. Tetrahedral coordination, with proportionately larger metal-ligand bond distances, is also observed on replacing the Cu by Hg(II) [43]. This suggests that the irregular geometry of the active site is imposed on the metal atom by the polypeptide... 

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