No abstract

A major factor influencing Co-C bond stability both in B12 compounds and organocobalt BI2 models appears to be the coordination number. Five-coordinate complexes are unusually stable to Co-C bond thermolysis, but structurally characterized examples of five-coordinate organocobalt complexes are rare. None are known for cases where R is a bulky group, such as i-C3H7. We report two definitive examples of five-coordinate complexes of the type Co(saloph)R, where saloph = dianion of disalicylidene-o-phenylenediamine. The structure of one species (R = i-C,H7) permits the first assessment of the effects of R bulk on the structure and also provides insight into the structural basis for the thermal stability of the species of reduced coordination number. We have also characterized the third example of a six-coordinate organocobalt saloph compound.Specifically, the crystal and molecular structures of Co(saloph)-i-C,H7-1.5H20 (I), Co(saloph)CH,.0.5H20 (11), and pyCo(saloph)CH2CF3 (py = pyridine) (111) are described. Compounds I and I11 are pentacoordinate and hexacoordinate, respectively, while crystals of I1 are comprised of both pentacoordinate and dimeric molecules. Crystallographic details follow. I: C23H21C~N202-1.5H20, space group Pbcn, a = 26.075 ( 6 ) A, b = 9.617 (3) A, c = 16.109 (3) A, Dcalcd = 1.46 g cm-,, Z = 8, R = 0.057 for 5437 measured reflections. 11: C21H17CoN202~0.5H20, space group P i , a = 12.100 (8) A, b = 20.392 (9) A, c = 11.606 (8) A, LY = 53.3 (l)', p = 127.6 (I)', y = 121.7 (l)', Dcald = 1.50 g ~m -~, Z = 4, R = 0.041 for 7360 measured reflections. 111: C27H21CoN302F3-0.5H20.0.5C5H5N, space group P21/c, a = 14.255 (8) A, b = 19.318 (8) A, c = 19.434 (8) A, 0 = 93.7 (I)', Dcalcd = 1.45 g ~m -~, Z = 8, R = 0.055 for 13588 measured reflections. Several interestingfeatures emerge from these structures. First, the Co atom in I and I1 is displaced toward R. Second, the Co-C bond is shorter than in hexacoordinate compounds, but steric strain remains for i-C,H7 even in the pentacoordinate species. Third, in the unusual example of both a dimeric and monomeric molecule in the same crystal in compound 11, the monomeric form seems to be stabilized by the water of crystallization. This water is hydrogen-bonded to the saloph ligand and not coordinated to Co. Dynamic IH NMR measurements of 3,5-lut exchange in 3,5-lutCo(saloph)R (where 3,5-lut = 3,5-lutidine, R = CH2CN, CH2CF,, or CH,) reveal that the rate of exchange increases more rapidly with increasing electron donation by R in the series CH2CN to CH2CF, to CH3 than in other classes of BI2 compounds. The saloph ligand has recently been shown to exhibit a large structural cis influence and a large kinetic cis effect compared to the other classes of models. It was also recently recognized that the trans and cis influences are synergistic. Thus, the dynamic NMR measurements provide evidence that the cis and trans kinetic effects are also synergistic. When R = CH,, for example, exchange of the neutral axial ligand in saloph complexes is approximately 8 X 1O' O ti...

No abstract

mercury s orbitals decrease in energy while the energies of the nonbonding iodide p orbitals change only slightly, giving rise to a decrease in the transition energy.The mechanism of the phase transition has been explained by LeDuc, et al.18 The transition is first order. The large mercury ions act as a valve prohibiting mobile silver (or copper) ions from disordering by blocking the conduction paths. When the thermal energy becomes high enough to allow the mercury ions to surmount their potential barriers, a large number of silver (or copper) ion carriers exist with high enough thermal energy to enter the vacant sites. Disordering then occurs rapidly. The optical data support this explanation. The nonlinear changes measured by this sensitive technique only occur over a small temperature range near the transition temperature.

No abstract