. Can. J. Chem. 69, 121 (1991).The crystal and molecular structures of s n ( % t -B~)~ and [Sn(&i-Pr)4.H&i-Pr]2 have been determined by single-crystal X-ray diffracti9n. Sn(&t-Bu)40crystallizes in the monoclinic crystal system with space group C2/c, where a = 17.382(6) A, [Sn(&i-Pr)4.H&i-Pr]2 exhibits an edge-shared, bi-octahedral structure in the solid state that is distorted due to the presence of asymmetric hydrogen bonding between axially coordinated alcohol ligands and an isopropoxide ligand. I3C NMR and IR spectroscopic data have been recorded for Sn(&t-Bu)4 and S n ( & t -B~-d~)~ to establish criteria for unambiguous identification of solution structures of tin(1V) alkoxides. It is demonstrated that the two-bond 2~~j C -&~~9 s n coupling constant is larger for terminal alkoxide ligands than for k2-alkoxide bridges, and the v(Sn-0) stretching frequency has been assigned. The dynamic solution behaviour of [Sn(&i-Pr)4.H&i-Pr]2 has been studied using variable temperature 'H and I3C NMR spectroscopy. The data obtained are consistent with a process that involves rapid reversible dissociation of isopropanol at room temperature. Upon cooling, the equilibrium concentration of the species with coordinated alcohol increases, and the molecule undergoes rapid intramolecular proton transfer (AG* < 11.9 kcal mol-I). Upon further cooling, the I3C NMR data are consistent with a solution structure analogous to that found in the solid state.Key words: tin, alkoxide, NMR, dynamic, structure. travers deux liaisons 2~~lC-&ttssn, est plus grande pour les ligands alcoolates terminaux que pour les ponts alcoolates-k2; de plus, on a attribut la frtquence de,. Faisant appel a la rmn du 'H et du I3c, a temperature variable, on a ttudit le comportement dynamique du [Sn(%i-Pr).HO--i-Pr] en solution. Les donntes obtenues sont en accord avec un processus qui implique une dissociation reversible rapide de l'isopropanol, a la temptrature ambiante. Par refroidissement, la concentration de l'espkce avec de I'alcool isopropylique augmente et la moltcule subit un transfert intramoltculaire rapide de proton (AG* < 1 1,9 kcal mol-I). Si on continue le refroidissement, les donntes de la rmn du I3c obtenues sont en accord avec une structure en solution qui serait analogue a celle qui est trouvte a l'ttait solide.

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ChemInform Abstract The reaction of the Cu halides (II) with the peptide ligands (I) which resemble part of the metal-chelating portion of the antitumor drug bleomycin affords the new title complexes (III) and (IV). (IIIa) crystallizes in P21/n with Z=4, (IVb) in P21/c with Z=4. In solution the compounds (III) dissociate into monomeric tetragonal Cu(II) complexes. Some magnetic properties of (III) and (IV) are discussed.

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. Can. J. Chem. 68, 565 (1990) The structural data show a relatively short approach between the Rh centres (2.948 A) and between the bridging sulfur atoms (2.888 A). The implications of these structural features in terms of metal-metal and sulfur-sulfur bonding are discussed. In addition, the implications of these results with respect to the formation of thiolato-bridged, early-late heterobimetallics is considered.Key words: thiolate abstraction, rhodium thiolate bridged dimer.

The sol-gel-type condensation of tin(1V) ethoxide [Sn(OEt),], (where OEt is ethoxide) under basic conditions produced spherical, submicrometer-sized tin(1V) oxide (cassiterite) particles. Transmission electron microscopy and powder X-ray diffraction data indicated that the grain size was approximately 20 to 30 A (2 to 3 nm). The mixed-metal alkoxide compound [ZnSn(OEt)s] was hydrolyzed under analogous conditions to give either spherical or octahedral submicrometer-sized crystalline particles of ZnSn(0H)r depending on the solvents used. These data demonstrated that the stoichiometry of the mixed-metal alkoxide precursor was retained during Condensation. Thermal treatment of ZnSn(OH)6 resulted in crystallization of ZnSn03 at approximately 676°C. At neutral pH, hydrolysis of [ZnSn(OEt)6] resulted in formation of a high surface area (261 m2/g) amorphous powder. [

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to a direct interference of the handle upon the bond formation step when the chain is located at a short distance from the iron atom. In spite of similar AG¡ values, Table I shows that the activation energy remains close to 4 kJ-moU1 for 02 whereas it is increased by nearly 20 kJ-mol"1 for CO with 5. On the contrary, A changes by more than 2 orders of magnitude for 02, but exhibits little variation for CO along the 3-5 series. Accordingly, the origin of the destabilization of transition state I is almost purely entropic for oxygen and predominantly enthalpic for carbon monoxide. This can be understood by considering the structural modifications that must take place during the binding process. A linear orientation of carbon monoxide should be accompanied at first by an increased interaction of this ligand with the chain. At the same time the porphyrin becomes more ruffled, leaving the chain farther away from the iron in order to accommodate CO linearly. Presumably, the shorter the handle length, the more pronounced the increase of distortion required in the binding process. Using the crystallographic data relative to the penta-and hexacoordinated complexes of 5, we calculated that the distance of the two central C atoms from the iron is increased from about 4.8 to 6.9 Á and that the ruffling distortion increases from 6 to 14°d uring the binding process. The energy that must be expended appears as a larger activation enthalpy in compounds 4 and 5 as compared to 3. Binding oxygen, on the contrary, implies a bent conformation.Although crystallographic data for the 02 complexes are not available, it does not seem necessary to assume an increase of ruffling in this case since the bent orientation would tend to release the steric constraints with the chain. The invariant activation enthalpy supports this view. The destabilization appears as entropy terms. The orientational freedom of oxygen is known to be strongly limited due to H-bonding with the amide protons. However the effect has been shown to be significant only in the bound state.8 Orientational constraint of a different nature, e.g. the avoiding of transient lateral interactions with the protons of the methylene groups, may play a specific role in the transition state.Thus, in addition to confirming that hybrid models also react via two sequential energy barriers, the present results indicate that steric discrimination against CO does not occur in either transition state since both are destabilized to the same extent for CO and 02. This is partly in agreement with the findings of Traylor et al.21 that steric effects are seen mainly in association rates and that those are changed equally for CO and 02. However, a previous discussion of the linear free energy relationships observed in various porphyrin series8 has shown that this behavior is not general and can only be expected for a strong central steric hindrance. Moreover, only a detailed thermodynamic study is able to reveal that the AG changes, although very close for 02 and CO, are brought about by the compl...