Volume 50, Issue 11

The reaction of (Me 3 Si) 2 NLi with SCl 2 and elemental sulfur or with Se 2 Cl 2 and elemental selenium leads to the formation of mixtures of bis[bis(trimethylsilyl)amino]suIfides and selenides [(Me 3 Si) 2 N] 2 E x (E = S or Se; x = 1 - 4 ). The reaction products were identified by mass spectroscopy as well as by 77 Se and 13 C NMR spectroscopy. The reaction of (Me 3 Si) 2 NH with S 2 Cl 2 produces [(Me 3 Si) 2 N] 2 S 3 as the main product with only traces of other aminosulfides. [(Me 3 Si) 2 N] 2 S 3 was purified by distillation under reduced pressure and identified by elem ental analysis, mass spectroscopy, and by 1 H, 13 C, and 14 N NMR spectroscopy. The successful cyclocondensation reaction of [(Me 3 Si) 2 N] 2 S 3 with SCl 2 and SO 2 Cl 2 produces S 4 N 2 in 72% yield and provides further verification of the identity of bis[bis(trimethylsilyl)amino]trisulfide. While the analogous reaction of (Me 3 Si) 2 NH with Se 2 Cl 2 also leads to the formation of [(Me 3 Si) 2 N] 2 Se 3 (48 mol %) as the main product, the reaction mixture contains [(Me 3 Si) 2 N] 2 Se 2 and [(Me 3 Si) 2 N] 2 Se 4 in significant amounts (17 and 35 mol %, respectively). Attempts to purify the mixture by distillation under reduced pressure resulted only in the formation of a 83:17 mixture of [(Me 3 Si) 2 N] 2 Se 2 and [(Me 3 Si) 2 N] 2 Se 3 .

Me 3 SnF (1) reacts with many hydrolyzable chlorides to give Me 3 SnCl and the corresponding fluoride. The formation of PhPF 2 , (ClCH 2 )MeSiF 2 , F 2 PCH 2 PF 2 and PF 5 is described. The reaction of triorganotin fluorides (Ph 3 SnF, Bu 3 SnF) with CaBr 2 yields pure triorganotin bromides. 1 was found to act either as a fluoride-acceptor or, towards PF 5, as a fluoride-donor.

The tetracupferronato-bis(μ-methoxy)-diiron(III) complex has been synthesized in acetonitrile and its crystal structure determined. C 26 H 26 N 8 O 10 Fe 2 , monoclinic, space group C2/c, a =18.425(3), b = 12.999(2), c = 15.046(3) Å, β = 124.66(2)°,V = 2964(1) Å 3 , Z = 4, wR(2) = 0.097 from 1491 reflections (F 2 ). Two iron(III) atoms are at special positions in six-fold coordination, bridged by O with a Fe-O-Fe angle of 102.4(1)° and a distance between two iron(III) centers of 3.075(1) Å. Temperature-dependent magnetic susceptibility measurements reveal a antiferromagnetic exchange interaction (J = -14 cm -1 ) between the iron(III) centers.

The reaction between [CH 2 =C(C 6 H 4 -4-NO 2 )CH 2 NMe 3 ]I and eight equivalents of BiI 3 in thf (tetrahydrofuran) afforded, after work-up, dark red crystals of the compound [CH 2 =C(C 6 H 4 -4-NO 2 )CH 2 NMe 3 ] n [{Bi 2 I 7 } n ], the structure of which was established by X -ray crystallography and shown to contain a polymeric iodobismuthate anion. An analogous reaction between [CH 3 (CH 2 ) 2 COS(CH 2 ) 2 N(CH 3 ) 3 ]I and eight equivalents of BiI 3 afforded orange crystals of the compound [CH 3 (CH 2 ) 2 COS(CH 2 ) 2 N(CH 3 ) 3 ] 4 [Bi 4 I 16 ] which was shown by X -ray crystallography to contain the tetranuclear [Bi 4 I 16 ] 4- anion.

The crystal structure of bis(3-phenyl-4-benzoyl-isoxazol-5-onato)copper(II) was determined for com parison with the copper complexes of 1-ghenyl-3-methyl-4-benzoyl-pyrazol-5-one and some β-diketones. Crystal data: a = 13,320(6) Å; b = 8,421(7) Å; c = 11,516(4) Å; β = 92,34(4)°; space group P2 1 /c ; Z = 2; R = 0,041; 2088 unique reflexions.

In the quest of suitable precursors for the synthesis of Mo(IV) carbonyl complexes the compounds [Mo(CO) 2 (PCy 3 )(′ bu S 4 ′)] (3) and [Mo(CO) 2 (PCy 3 )(′S 4 ′)] (4) [′ bu S 4 ′ 2- = 1,2-bis(2-mercapto-3,5-di-t-butyl-phenylthio)ethane(2-); ′S 4 ′ 2- = 1,2-bis(2-mercaptophenylthio)ethane(2-)] were synthesized via CO substitution from [Mo(CO) 3 (′ bu S 4 ′)] (13) and [Mo(CO) 3 (′S 4 ′)](14). 4 was characterized by X -ray structure analysis and exhibits, due to the sterically demanding PCy 3 , a strongly compressed [Mo(′S 4 ′)] core. Intramolecular redox reactions of 3, 4 and, in addition, [Mo(CO) 2 (PMe 3 )(′S 4 ′)] (15) leading to thioether dealkylation via loss of C 2 H 4 from the ′ R S 4 ′ 2- ligands and Mo(II)→ Mo(IV) oxidation yielded the Mo(IV) complexes [Mo(CO)(PCy 3 )(′ bu S 2 ′) 2 ] (6), [Mo(CO)-(PCy 3 )(′S 2 ′) 2 ] (7) and [Mo(CO)(PMe 3 )(′S 2 ′) 2 ] (8) [′ bu S 2 ′) 2 ]= 3,5-di-t-butyl-1,2-benzenedithiolate(2-), ′S 2 ′ 2- = 1,2-benzenedithiolate(2-)]. Depending on the nature of the phosphines and the R substituents of the ′ R S 4 ′ 2- ligands, the syntheses required very specific reaction conditions ranging from thermolysis in vacuo to heating in boiling THF. For instance, [Mo(CO) 2 (PCy 3 )(′S 4 ′)] (4) decomposes in CH 2 Cl 2 solution even at ambient temperature yielding the binuclear Mo(II) complex [Mo(CO)(μ-′S 4 ′)] 2 (10), which was characterized by an X-ray structure determination. Thermolysis of 4 in vacuo at 80 °C , however, results in the formation of the Mo(IV) complex 7. By optimizing the reaction conditions preparative amounts of the prototype [Mo(CO) 2 (′ bu S 2 ′) 2 ] (1) have now also become accessible. These findings can be rationalized in part on the basis of results obtained from cyclic voltammetry. The redox potentials of the Mo(IV) complexes are drastically shifted by about -0.5 V per substitution of CO by PR 3 , and the substitution of ′S 2 ′ 2- by ′ bu S 2 ′ 2- ligands causes a negative shift in the range from 0.03 V to 0.18 V. The results of IR spectroscopical m onitoring the substitution of CO by PMe 3 in [Mo(CO)(PMe 3 )(′ R S 2 ′) 2 ] yielding [Mo(PMe 3 ) 2 (′ R S 2 ′) 2 ] suggested an associative substitution mechanism via the intermediate [Mo(CO)(PMe 3 ) 2 (′ R S 2 ′) 2 ]. The formation of [Mo(CO)-(PMe 3 )(′ bu S 2 ′) 2 ] (12) from [Mo(PMe 3 ) 2 (′ bu S 2 ′) 2 ] (9) and CO under standard conditions further shows that the direct coordination of CO to Mo(IV ) centers is also possible if these centers are surrounded by thiolate donors. The results are discussed with respect to the reactivity of the Mo centers of the FeMo cofactors in nitrogenases.

Single crystals of Pb 3 [(PO 4 ) 2 BPO 4 ] were obtained by melting B 2 O 3 , P 2 O 3 and PbO at 960 °C. The structure is related to the Ba 3 [(PO 4 ) 2 BPO 4 ] type. X-ray investigations on single crystals led to the space group Pbca - D 15 2h (Nr. 61) with a = 694.6(1), b = 1419.9(3), c = 2111.6(2) pm, Z = 8 . Boron and phosphorus are tetrahedrally coordinated by oxygen. The polyhedra form one-dimensional infinite helices that contain two additional PO 4 groups connected to the BO 4 -unit. Lead is eightfold coordinated by oxygen. The relation to the crystal structure of Ba 3 [(PO 4 ) 2 BPO 4 ] is discussed.

The reaction of cyclohexylamine, sulfur, and hydrogen sulfide in a 2:1-mixture of ethanol and water gives a red cyclohexylammonium polysulfide solution. Orange crystals of the title compound can be obtained by slow cooling. The compound crystallizes in the centrosymmetric triclinic space group P1̄. The lattice constants are a = 8.478(7) Å , b = 11.97(1) Å , c = 14.141(8) Å, α = 102.60(6)°, β = 90.53(5)°, γ = 90.43(7)°. The S 7 2- -chain has an uncommon cis-trans-cis conformation with a sequence of the signs of torsion angles of (+--+) and (-++-).

Measurements of the 57 Fe Mössbauer effect and of magnetic susceptibilities have been performed on the 57 Fe doped or pure quasi-1-d antiferromagnetic chain compound enH 2 MnF 5 as a function of temperature. Particular attention was paid to the regions very near the Néel point at T N = 14.5(5) K. The Mössbauer spectra fitted by the Blume-Tjon model show definite relaxation effects, which are attributed to short-range order with temperature-dependent relaxation times. The soliton model of non-linear excitations was applied. Experimental data confirm the predicted exponential temperature dependence of the thermal excitation of moving domain walls. From the activation energy E A /k = 184(5) K and the 1-d exchange energy J/k = -13.6(2) K a local anisotropy energy D/k of -2.4 (3) K was obtained which is smaller than D/k = -3.5 K derived from our single crystal measurements of magnetic susceptibilities. Within the 3-d magnetic ordering the interchain interaction amounts to J′/k ≈ 0.22(4) K (J′/J = 15.8 10 -3 ). A zero-spin reduction ⊿S/S equal to 17% was obtained. 1-d correlations are observed for T N <T< |J|S (S + 1)/k ≈ 78 K, and a crystallographic phase transition at 216 K was indicated in the magnetic properties as well as by DSC and X-ray powder diffraction measurements.

Reaction of triphos (CH 3 C(CH 2 P(Ph) 2 ) 3 ) with Co (II) aq (BF 4 ) 2 followed by reaction with aminoacids leads to the cationic complexes 1-6 ([triphosCoNH 2 CH(R)COO] + (R = H; Gly (1), R = methyl: L-Ala (S-2), D-Ala (R-2), R = isopropyl: L-Val (3), R = N -propyl: L - Pro (4), R = phenyl: D-phenylglycine (5), R = benzyl: L - Phe (6 )), which can be isolated in good yields by crystallization as their B(Ph) 4 - salts. The analogous reaction of the modified tripod ligand PhCH 2 C(CH 2 P(Ph) 2 ) 3 with L-alanine leads to the complex [PhCH 2 C(CH 2 P(Ph) 2 ) 3 Co(L-Ala)] + , 7. The cations 1-7 have been characterized by spectroscopic methods and their structures determined by X -ray analyses which unequivocally demonstrate pentacoordinated Co (II) centers.

Single crystals of Co 1.5 Ti 0.5 (BO 3 )O (I) and Co 1.5 Zr 0.5 (BO 3 )O (II) were obtained by a B 2 O 3 flux technique. Both compounds crystallize with orthorhombic symmetry, space group D 16 2h -Pnma (Nr. 62), I a = 928.1; b = 311.1; c = 940.1 pm; Z = 4 and II a = 949.5; b = 323.42; c = 934.7 pm; Z = 4. The compounds are isotypic to the mineral warwickite. All metal ions are octahedrally coordinated by oxygen ions. The structure contains isolated, trigonal planar BO 3 units and oxygen that is not coordinated to boron.

Lithium indolide and lithium pyrrolide react with fluoro- and chlorosilanes with formation of mono- and bis(indol-1-yl)- and -(pyrrol-1-yl)silanes, tris- and tetrakis(pyrrol-1-yl)silanes as well as of silanes substituted with different heteroaromatic systems. The crystal structures of the lithium indolide and the tetrakis(pyrrol-1-yl)silane have been determined and are discussed.

Algebraic equations for transition frequencies and intensities are derived for the AXX′ 2 spin system. In favourable cases the resonance frequency ν A and all coupling parameters J AX, J AX′ and J XX′ can be extracted from the A-part of the AXX′ 2 spectrum. Examples of deceptively simple spectra are discussed.

Potassium-graphite (C 8 K) reduces phenyltrichlorosilane to give polyphenylsilynes in high yields. Branched copolysilanes have been prepared via reductive coupling of phenyltrichlorosilane and diorganodichlorosilanes with C 8 K. Polymers of this type are also obtained by reductive coupling of triflate substituted polysilanes with C 8 K. The soluble polymers and copolymers with molecular weights in the range ≈ 4000 to 10000 have been characterized by 29 Si-NMR spectroscopy.

N,N-diethyl-Nʹ-ferrocenoyl-thiourea and N-(morpholino-thiocarbonyl)ferrocenecarboxylic amide have been prepared by the reaction of ferrocenoyl chloride with potassium thiocyanate and the respective amine in dry acetone. These bidentate ligands yield neutral heterometalIic complexes with Ni(II), Cu(II), Mn(II) and Co(III). The dark brown air stable crystals of bis-(N,N-diethyl-Nʹ-ferrocenoylthioureato)nickel(II) were characterized by X-ray structure de­termination. Lattice dimensions: a = 1870.9(1), 6= 1161.5(1), c = 1491.4(1) pm; space group Pca2 1 , Z = 4, R = 0.030 for 5707 observed reflections.

The compounds EtPh 3 PI 3 , EtPh 3 PI 5 and EtPh 3 PI 7 have been prepared by the stoichiometric reaction of EtPh 3 PI with iodine in a mixture of dichloromethane and ethanol. EtPh 3 PI 3 cry­stallizes in the monoclinic space group P2 1 /n with a = 1176.7(3) pm, b = 1202.5(3) pm, c = 1565.9(4) pm, β=93.22(2)° and Z = 4. The crystal structure has been refined to R F = 0.043 for 3394 reflections. EtPh 3 PI 5 crystallizes in the monoclinic space group I2/a with a = 1913.9(4) pm, b = 1239.0(2)pm, c = 2435.1(4) pm, β = 115.84(1)° and Z = 8. The crystal structure has been refined to R F =0.052 for 3043 reflections. EtPh 3 PI 7 crystallizes in the orthorhombic space group Pbca with a = 1702.7(4)pm. b = 1952.5(4) pm, c = 1809.8(4) pm und Z = 8. The crystal structure has been refined to R F = 0.053 for 3208 reflections. The structures may be described as layerlike packings of cations Ph 3 EtP + and of anions I - 3 , I - 5 and I - 7 , respectively. The isolated triiodide ion I - 3 is slightly asymmetrical and slightly bent as expected. The pentaiodide ion I - 5 forms iodine ribbons like a rope ladder. The heptaiodide ion I - 7 is composed of pentaiodide groups and iodine molecules connected to puckered anionic layers.

The new dinuclear [1.1]metallocenophanes, bis(dimethylsila)-[1.1]ferrocenophane (2a) and bis(dimethylsila)-[1.1]ruthenocenophane (2b), were synthesized by the reaction of dilithium di(cyclopentadienyl)dimethylsilane with FeCL·2 thf and RuCl 2 ·4 dmso in tetrahydrofuran solution, and compared with the known bis(methylene)-bridged analogues [1.1]ferrocenophane (8a) and [1.1]ruthenocenophane (8b), respectively. A series of mononuclear [n]ruthenocenophanes including 1,1,2,2 -tetramethyl-[2]ruthenocenophane (10b), 1,2 - bis(dimethylsila)-[2]ruthenocenophane (3b), 1,3-bis(dimethylsila)-2-oxa-[3]ruthenocenophane (4b) and 1,4-bis(dimethylsila)-[4]ruthenocenophane (5b) was also prepared and studied together with the corresponding [n]ferrocenophanes, 10a and 3a-5a. 1,1′-Dilithioruthenocene was found to be a versatile starting material for the synthesis of silicon-substituted ruthenocene derivates such as Ru(C 5 H 4 -R) 2 (R = SiMe 3 (6b), Si 2 Me 5 (7b)) and [n]ruthenocenophanes (3b- 5b). A complete set of 1 H, 13 C and 29 Si NMR data is presented, and the available structural data are discussed with respect to the tilt angle a of the cyclopentadienyl ring ligands which is larger in the more strained [2]ruthenocenophanes than in the corresponding [2]ferrocenophanes.

Semiquinone radical anions are prototype compounds for contact ion pair formation with metal counter cations. In order to investigate the still open question whether bulky alkyl groups can sterically interfere, diphenoquinone derivatives O=C(RC=CH) 2 C=C(HC=CR) 2 C=O with R = C(CH 3 ) 3 , CH(CH 3 ) 2 and CH 3 have been selected and the following ESR/ENDOR results are obtained for the alkaline metal cations: The tetrakis(tert-butyl)-substituted radical anion only adds Li ⊕ and Na ⊕ , while K ⊕ forms no ion pair. The 3,3ʹ,5,5ʹ-tetra(isopropyl)diphenoquinone radical anion is accessible to all cations Me ⊕ , although Rb ⊕ and Cs ⊕ seem to be present solvent-separated in solution. The tetramethyl-substituted radical anion unfortunately polymerizes rapidly. Additional information concerns the ESR/ENDOR proof for ion triple radical cation formation [Li ⊕ M •⊖ Li ⊕ ] •⊕ , or the difference in the coupling constants upon Me ⊕ docking at one δ⊖ O=C group, suggesting that about 87% of the spin density is located in the cation-free molecular half of the diphenoquinone radical anion. Based on the wealth of ESR/ENDOR information, crystallization of the contact ion pairs and their structural characterization should be attempted.

The electrode reactions at platinum anodes in 1.5 M H 2 SO 4 + 2.3 M (NH 4 ) 2 SO 4 solution with and without addition of NaF (0.5 M) have been examined. Impedance spectra between 1 to 100.000 Hz were measured under galvanostatic conditions. Electrolyte, polarization, chargetransfer resistance (R D ), double-layer capacitance (C D ) and inductance (L) were determined. For a constant current density, fluoride causes an increase o f C D and a decrease of R D . At a certain potential (measured and corrected by the electrolyte resistance) C D nearly remains the same with and without fluoride, whereas R D is slightly increased by fluoride. This behaviour is explained by the double function o f the fluoride: It is adsorbed on the anode surface and causes an increase o f the real current density on the surface. Simultaneously, the evolution o f oxygen is hindered by fluoride.

The 31 P suspension NMR spectra o f silica-immobilized bifunctional phosphines and their di- and tricarbonylnickel complexes were recorded using conventional high resolution NMR equipment. The practical advantages of suspension NMR spectroscopy, as compared to 31 P CP/MAS NMR spectroscopy, are discussed. General tendencies are demonstrated: The suspension NMR signals become broader with increasing solvent viscosity and decreasing solvent polarity. Phenyl groups as spacers lead to broader resonances than alkyl chains. The linewidths increase in the case of chelation.

Reaction of meso-[Ru(L)(′ bu S 5 ′)] complexes (L = CO (1), PPh 3 , PCy 3 ) with one equivalent of oxonium salts R 3 OBF 4 (R = CH 3 , C 2 H 5 ) yields the C 1 symmetrical monoalkyl derivatives [Ru(L)(′ bu S 5 ′-R)]BF 4 (L = CO, R = CH 3 (2), C 2 H 5 (3); L = PPh 3 , R = CH 3 (5); L = PCy 3 , R = CH 3 (6)), in which one of the thiolate donors of the starting complexes is alkylated diastereospecifically. Monoalkylation of the binuclear complex [Ru(′ bu S 5 ′)] 2 leads to C 1 symmetrical [Ru(′ bu S 5 ′-CH 3 )] 2 (BF 4 ) 2 (7) which gives the hydrazine complex [μ-N 2 H 4 {Ru(′ bu S 5 ′-CH 3 )} 2 ](BF 4 ) 2 ·N 2 H 4 (9) upon reaction with N 2 H 4 . Reaction of 1 with 1.5 equivalents of CF 3 SO 3 H yields the isolable thiol complex [Ru(CO)(′ bu S 5 ′-H)]CF 3 SO 3 ·0.5 CF 3 SO 3 H (4). The 1 H-NMR spectra are temperature-dependent and indicate exchange of the thiol proton between the thiolate donors of the ′ bu S 5 ′ 2- ligand. Twofold protonation of 1 could only be proved IR-spectroscopically, dialkylation of 1, however, yields isolable [Ru(CO)(′ bu S 5 ′-(CH 3 ) 2 )](BF 4 ) 2 (8) as a mixture of diastereomers. The IR stretching frequencies ν(CO) of the CO complexes indicate that the electron densities at the ruthenium centers are decreased and the Ru-L bonds are weakened. However, the ligands L in the [Ru(L)(′ bu S 5 ′-R)]BF 4 complexes do not exchange more readily than in the parent complexes.

A series of paramagnetic (S = 3/2) chromium half-sandwiches of the type CpCrX 2 D (D = donor) were synthesized by starting from [CpCrX 2 ] 2 , Cp 2 Cr, CpCrCl 2 (THF), and Cp*Cr(CH 3 ) 2 [P(CH 3 ) 3 ]. Besides the parent cyclopentadienyl (Cp) the alkylated derivatives CH 3 C 5 H 4 , (CH 3 ) 5 C 5 (Cp*), and C 2 H 5 (CH 3 ) 4 C 5 were bound to chromium. The electronegative substituent was X = F, Cl, Br, I, and triflate, while the donors were three ethers, four ketones, dimethylsulfoxide, acetonitrile, methylisocyanide, pyridine, and seven molecules ER 3 where E = N, P, As, Sb. The half-sandwiches were partly isolated and partly established in solution. The 13 C and 1 H NMR spectra showed strongly shifted signals which allowed to quantitatively investigate the equilibrium between CpCrX 2 D and the anti ferromagnetic species [CpCrX 2 ] 2 . The NMR signals of CpCrX 2 D and its substituted derivatives appeared in characteristic ranges thus providing a means of rapid identification. Considerable spin density was found to be induced on the ligands; it is negative in the Cp π system. As for the donors, inter- and intramolecular dynamic behavior as well as selective spin transfer to the γ protons of acetonitrile, methylisocyanide, and ketones was detected.

The dinuclear rhenium(IV) complexes with the C 3 S 4 Se-ligand [1,3-dithiole-2-selone-4,5- dithiolate(2-)], [PPh 4 ] 2 - (1) and [NEt 4 ] 2 [Re 2 (C 3 S 4 Se) 5 ] (2), were prepared. Complex 2 was oxidized by a reaction with [Fe(C 5 H 5 ) 2 ][PF6] or [TTF] 3 [BF 4 ] 2 (TTF· + = the tetrathiafulvalenium radical cation) and by the current-controlled electrolysis to yield [NEt 4 ] x [Re 2 (C 3 S 4 Se) 5 ] (x = 0.15 and 0.5) and [TTF][Re 2 (C 3 S 4 Se) 5 ]. The compounds exhibit electrical conductivities of 3.3 x 10 -3 - 9.3 x 10 -4 S cm -1 for compact pellets at room temperature. Electrochemical and spectroscopic properties of the complexes are discussed on the basis of cyclic voltammetry and ESR, XPS and IR spectra as well as the magnetic susceptibility of the TTF salt.

The interaction of adsorbed acetonitrile with microporous [Si,Al]-ZSM-5 and [Si,B]-ZSM-5 was investigated by solid state NMR. 15 N-CP MAS NMR of the system H-[Si,Al]-ZSM-5 / acetonitrile clearly shows the Bronsted and Lewis acid site interactions. Different coordination types of the complex ZSM-5/MeCN are considered. The accessibility of the acid sites of [Si,B]-ZSM-5, a material structurally analogous to zeolite ZSM-5, was studied by 11 B-MAS NMR. The complexation of acetonitrile at the BO 3 sites gives a narrow line at -4 ppm. In contrast to hydration phenomena of ZSM-5, the experiments show that acetonitrile is not capable of interacting with all of the BO 3 sites of the borosilicate, presumably due to steric effects.

The reaction of cis-Me 2 Os(CO) 4 with HBF 4 ·Et 2 O affords the complex cis- (OC) 4 Os(FBF 3 )CH 3 (2). The coordinated tetrafluoroborate of 2 is easily substituted by H 2 O, [Re(CO) 5 ] - , [Re(CO) 4 (CN)CH 3 ] - (from Re(CO) 5 CH 3 and NaN(SiMe 3 ) 2 ), SCN - and PPhMe 2 to give neutral or cationic complexes 4-6, 8, 9, respectively. The structure of cis-H 3 C(OC) 4 Os- Re(CO) 5 has been determined by X-ray diffraction.

The two rotamers (head-tail, 1, and head-head, 2) of the bis(1-methylcytosine)complex of trans-(CH 3 NH 2 ) 2 Pt(II), have been crystallized as ClO 4 - (1, 2a) and PF 6 - (2b) salts and characterized by X-ray crystal structure analysis and 1 H and 195 Pt NMR spectroscopy. In aqueous solution, 1 is preferred over 2 by 70:30. Upon slow crystallization from H 2 O, 1 is obtained as the only product. Isolation of 2a and 2b has now been accomplished via formation of the heteronuclear derivative trans-[(CH 3 NH 2 ) 2 Pt(1-MeC - -N3,N4) 2 Hg] 2+ , in which the deprotonated 1-methylcytosinato ligands (1-MeC - ) are oriented head-head, precipitation of Hg(II) by thiourea, and rapid crystallization of the parent compound. The solid state structures of 1 and 2b differ markedly in a number of aspects. Isolation of pure 1 and 2 permits a detailed study of the kinetics and thermodynamics of the interconversion of the two rotamers. From comparison with the behavior of 1 and 2 in H 2 O on the one hand and DMSO and DMF on the other a clear solvent effect on the rotamer distribution is seen which most likely relates to differences in H bonding between solvent and solute.