Polynuclear linear

ISOCYANIDES AND NITRILE LIGANDS IN POLYNUCLEAR LINEAR COMPLEXES... 

Aoki and Chen examined the statistical thermodynamics of polynuclear linear complexes with mixed-valence states by means of the theory of correlated walk. Expressions for the equilibrium electrode potential of linear V-nuclear complexes with homoredox centers were derived as a function of the molar fraction of the oxidized moiety, the nearest-neighbor interactions and N. Under conditions of large interaction energy, two-, three- and four-nuclear complexes were predicted to have two, three, and four voltammetric peaks, respectively. For odd number of A> 3, only three peaks are predicted while for even number N>4, only four peaks are predicted. In cases where 00, the number of peaks reduces to two. 

Silver ions form a number of complexes with both TT-bonding and non-TT-bonding ligands. Linear polynuclear complexes are known. The usual species are AgL and AgL2, but silver complexes up to AgL have been identified. Many of these complexes have commercial appHcation. 

Figure 11.13 Structures of polynuclear nitrosyl complexes (a) ((Cr( j -C5H5)(NO))2(M2-NH2)(M2-NO) showing linear-terminal and doubly bridging NO and (b) (Mn3( j -C5H5)3( 3-NO)3( 3-NO)] showing double-and triply-bridging NO the molecule has virtual symmetry and the average Mn-Mn distance is 250 pm (range 247-257 pm).

This material, previously thought to be Hg CAsP Ij, contains infinite linear chains of Hg atoms in two mutually perpendicular directions. Each Hg atom bears a charge of 0.35. On further reaction of AsFj with Hg in liq SO2, a deep red solution is obtained as well as the golden crystals. From this red solution two further polynuclear Hg derivatives, HgjfAsF Ij and Hg4(AsF5)2 crystallize ... 

Linear Polynuclear Lanthanide Complexes Showing Robust SMM Behaviour... 

Polynuclear Lanthanide Single Molecule Magnets 3.2.2.1 Linear Dy3 SMMs... 

Arrhenius plot (cf. Figure 5.2b) features two linear regions, which can be fitted to yield A1 = 11.0 cm-1 (r0 = 7.7 X 10-4 s) and A2 = 82.1 cm-1 (t0 = 6.2 X 10-7 s) for the temperature regimes 1.6-8 and 18-22 K, respectively. This behaviour has been attributed to distinct Dy3+ single-ion and polynuclear Con2Dyln2 effects where the A barrier is believed to originate from the coupled system and the barrier A2 from the localized LF excitations of the Dy3 ions. 

In the following section, attention is directed towards these linear polynuclear Fe(II) spin crossover systems, whereas subsequent sections focus on mononuclear Fe(II) spin transition compounds containing chelating 1,2,4-triazole derivatives. 

The demonstration of the formation of a hexanuclear zinc complex with the S-donor ligand 2-aminoethanethiolate, containing Zn3S3 and Zn4S4 cyclic units, contributes to the building up of a pattern of polynuclear complex formation based on coordination preferences of the metal ions involved (320) - reaction of Zn2+ with salicylideneamino ligands and pyrazine can give linear tetranuclear complexes (321). Another hexanuclear zinc complex appears in the section on supramolecular chemistry below (Section VII.D). 

Most helicates have linear axes, though a few helicates with circular axes are known - indeed the chiral (D4) molecular squares formed from Zn2+ and 2,5 -bis(2,2 -bipyridin- 6 -yl)pyrazine, 22, may be regarded as circular helicates (450). The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2/-diimine systems with their substitutionally-inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates of this type is believed to entail a kinetically favored triple helicate intermediate (484). Self-assembly of chiral-twisted iron(III)-porphyrin dimers into extended polynuclear species takes place through the intermediacy... 

The water-gas shift rates are obviously much lower when heterogenized in comparison with the Rh complexes in homogeneous solutions of the amines (also see Tables 30-33). Kinetics for nitrobenzene reduction were performed for the cis-[Rh(CO)2(2-picoline)2]PF6 catalyst, and reported in 2000. Kinetics displayed a first order dependence on Pco over the range 0-1.9 atm in the temperature range 80-120 °C. As with the kinetics previously reported by Lima Neto and coworkers,121 it was suggested that the CO addition preceded the rate limiting step. A non-linear dependence on the rate versus Rh concentration, as with the previous study, suggested participation by both mononuclear and polynuclear species. 

Isocyanides are long known as ligands able to afford linear polynuclear complexes. This is, for example, the case of the trinuclear [Rh3 (C6H5CH2NC)12I2]3 +, 46 and tetranuclear [Rh4 NC(CH2)3NC 8Cl]5 +, 47 complexes, the electrochemical behaviour of which is, however, unknown. 

---