Dimers hydroxo-bridged

Baraniak, E., Bruce, R.S.L., Freeman, H.C., Hair, N.J., and James, J. (1976) Structure and properties of a dimeric hydroxo-bridged ytterbium(III) complex di-p,-hydroxo-bis[triaquopyridine-2-carboxaldehyde-2/-pyridylhydrazoneytterbium(lll)] tetrachloride tetrahydrate. Inorganic Chemistry, 15, 2226-2230. 

The general reluctance of reagents to reduce O2 in one-equivalent states is further exemplified in the cases of V(II) and Cr(II). Autoxidation of V(1I), ultimately to V(III), produces ° an intermediate dimer, VOV ", identified spectrally and a known product of the reaction between V(II) and Clearly one path involves an initial two-equivalent oxidation of V(ri) to V(IV) and Swinehart ° calculates that 60 % of the oxidation follows this route. Cr(Il) perchlorate produces a species containing two Cr(III) species linked by one 0x0-or two hydroxo-bridges and Cr(lV) is proposed as the first intermediate F... 

Adventitious water is responsible for the formation of the dimeric hydroxo complex 31 obtained by reaction of AuCb with 1,4-dilithiotetraphenylbutadiene in ether solution [97[. The hydroxo-bridged complex [Au(C6H4N02-2)2( i-0H)[2 (32) was obtained either by reaction of Na[Au(C6H4N02-2)2(0Ph)2] with traces of water in CH2Cl2/n-hexane solution or by treatment of the dichloroaurated complex with NaOH [98[. The crystal structure of 32 2Et20 shows that it is a centrosymmetric... 

The hydroxo complexes [Au(6-Rbipy)(OH)2], postulated as intermediates in the formation of 44c-i, are not isolated nevertheless, in all the mass spectra (FAB conditions) a weak peak is found corresponding to these species. Unchanged 44c-i are quantitatively recovered from the reaction with aqueous solutions of HX (X = BF4 or PFfi) neither dihydroxo complexes similar to those observed in the vapor phase, nor hydroxo bridged dimers are obtained [25]. 

Sequential steps are proposed (75- 77) with two monomeric units combining to form first a single hydroxo-bridged dimer, which in turn closes to give the double hydroxo-bridged dimer. This latter species [(H20)4Cr(0H)2Cr(0H2)4]4+ is one of the major products from the 02 oxidation of [Cr(OH2)6]2+ (11) (Section I,A), and the crystal structure of the p-toluenesulfonate salt has been determined (78). 

In all these condensation processes there are a number of monomer species, related to one another by acid-base equilibria, that can combine to give the observed product. Thus, all binary combinations of [Cr(OH2)6]3+, [Cr(OH)(OH2)5]2+, [Cr(OH)2(OH2)4]+, and [Cr(OH)3 (OH2)3]° need to be considered (75) when forming the hydroxo-bridged dimer. Even though some of the lower-charged species are only present at low concentrations, the reactivity increases with deprotonation. In fact, the oligomerization processes (monomer - dimer dimer —> tetra-mer) are dominated by six reaction pathways (Table II), and each deprotonation step affords a rate enhancement. 

Metal-oxygen and metal-metal distances have been established for many oxo- and hydroxo-bridged dimers and polynuclear cations, for example (57)220 and several of type (58).217 Early work was on solids, but recently X-ray diffraction studies have established the geometries of a few such species in solution. EXAFS has proved valuable in establishing the geometry of the various di- and tri-nuclear forms of the various oxidation states of molybdenum in aqueous solution.23... 

Trivalent yttrium and lanthanide metals, except for promethium, have been complexed to octaethylporphyrin by heating at 210 °C in an imidazole melt.17 The complexes obtained as hydroxides, Mm(OEP)(OH), are unstable in acidic media. As the charge radius ratio rule predicts, the early lanthanide metalloporphyrins, MIU(OEP)(OH) (M = La, Ce, PR, Nd), are demetallated during purification, and the middle series (M = Sm, Eu, Gd, Tb, Dy) in 1 % acetic acid in methanol, while the last five (M = Ho, Er, Tm, Yb, Lu) survive in 2% acetic acid in methanol but are dissociated in dilute hydrochloric acid. The Mnl(OEP)(OH) appears to coordinate more than one equivalent of pyridine and piperidine, and dimerizes in noncoordinating solvents such as benzene and dichloromethane at 10 4 M concentration. The dimer is considered to be a di-p-hydroxo-bridged species, different from the p-oxo dimer, Scin(OEP) 20 (Scheme 6). 

Read, M.C., Glaser, J., Sandstrom, M. and Toth, I. (1992b) Hydrolytic oligomers of rhodium(III) a multinuclear NMR study of the doubly hydroxo-bridged dimer and trimer in aqueous solution. Inorg. Chem., 31, 4155-4159. 

The purple acid phosphatases (PAPs) are a class of phosphoprotein phosphatases which possess a p-oxo(hydroxo)-bridged dinuclear iron centre. An enzyme has been isolated from beef spleen which is purple in colour, while a violet phosphatase has been characterised from red kidney beans (KBPase). This latter enzyme consists of two subunits with M = 58200 and contains two equivalents of Zn(II) and Fe(III) per dimer which are essential for catalytic activity. KBPase hydrolyses nucleosidetriphos-phates as well as activated phosphomonoesters such as 4-nitrophenylphosphate or a-naphthyl phosphate (Beck et al., 1986). As with the beef spleen enzyme, KBPase is inhibited by tetrahedral oxoanions such PO and AsO . 

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