Mononuclear tetragonal

The catalytic site of CAO contains a mononuclear, tetragonal [Cu(His)3(OH2)J (x= 1 or 2) center in its resting state, in close proximity to the protein-bound cofactor 2,4,5-trihydroxyphenyl-... 

Recently [57], a computer has been used for the mutual subtraction of the experimental spectra associated with different copper coneentrations, and three types of individual signals have been identified in the poly(methacrylic-Cu(II)) system (Table 3.3 and Fig. 3-7). The ESR spectrum of complex I ([Cu]/[COOH] = 1 60) is a weak asymmetric signal characteristic of mononuclear tetragonal Cu(II) compounds with the structures A and B shown in Fig. 3.8. 

A crystal-structure determination on [Ni(PhCH2CS2)2] showed evidence of a Ni-Ni bond (Ni—Ni distance, 256 pm) in a bridging, acetate-cage, binuclear complex (363). Each nickel atom is 5-coordinate and is in a tetragonally distorted, square-pyramid spectroscopic evidence for a Ni-Ni bond has been obtained (364). The polarized crystal spectra showed more bands than predicted for a mononuclear, diamagnetic, square-planar nickel(Il), and the spectra are indicative of substantial overlap of the d-orbitals between the two nickel atoms. The bis(dithiobenzation)nickeKII) complex was found to exhibit unusual spectrochemical behavior (365). 

The coordination chemistry of Cr can be subdivided into the realm of mononuclear coordination compounds, which can be derived (at least formally) from the blue [Cr(OH2)e] ion, and the chemistry of metal-metal bonded species (analogs of Cr2(OAc)4). Most of the former are octahedral high-spin complexes with distinct tetragonal distortions caused by the Jahn - Teller Effect. With strong-field ligands, low-spin octahedral complexes are formed, and there are also some four-coordinate complexes of various coordination geometries. The metal-metal bonded species are described in Section 5. 

GHz). The spectral similarity between the type 2 Cu(II) in MCOs and in other mononuclear type 2 Cu(n) proteins is somewhat surprising since the coordination in the former is different than in the latter. While type 2 Cu(n) is most commonly characterized by a four-coordinate, tetragonally distorted square-planar ligand field (with, in some cases, some rhombic distortion also), the MCO type 1 Cu(II) is coordinated by two protein His residues and a solvent H2O only. The ligand field associated with this coordination is not directly obvious that is, among other uncertainties, whether the H2O is an equatorial or an axial ligand has not been estabhshed. In AO, the Cu-0 distance is 2.02-2.03 whereas in... 

D/3M contains two coppers per monomer and these Cu atoms are not equivalent. Based on spectroscopic data, it has been proposed that the oxidized enzyme contains two tetragonal Cu(II) centers with no more than three histidine ligands per Cu. As already mentioned, the Cu sites are inequivalent with CuH(His)3(H20)... Cu]vi(His)2X(H20) coordination. The identity of X was unknown. Reduction by ascorbate causes loss of the bound water molecules since water is a poor ligand for Cu. This leaves two 3-coordinate centers, one of which, Cum, now coordinates a S from methionine. Alternatively, this S could already be present in the oxidized form as a weakly bound axial ligand. The mononuclearity of the Cu centers requires Cuh to be at some distance from Cum and consequently must serve to shuttle electrons into the hydroxylating site. Cuf4 has been identified as the CO, and by inference, 02-binding site. ... 

Since copper enzymes are involved in redox processes, it was pertinent to explore Cu" complexes of 87 as the same with 54 were less stable. The first such stable complex 91 formed from 87 and Cu° perchlorate constituted a typical mononuclear five coordinate species with three imidazole groups and two water molecules in the coordination sphere. The small sized alkylnitriles displaced water molecules and 92, formed in acetonitrile, in its X-ray structure revealed a distorted tetragonal N4O environment provided by the N3 ligand, an acetonitrile buried inside the cavity and a water molecule placed outside (Figure 4 2000IC3436). 

A small number of oxo-alkoxides of chromium have been structurally characterized. The mononuclear chromium(vi) compounds [Cr(0)2Cl(0CPh3)] and [Cr(0)2(0CPh3)2] both involve tetrahedrally coordinated chromium. Chromium(v) is five-coordinated (tetragonal pyramidal) with the terminal oxo-ligand in the apical position in the mononuclear [Cr(0) 02C2(CF3)4 2] and binuclear [Cr2 (0)2 (m-0)2 02C2 (CF3 )4 2] auions. ... 

---