The first example of spin crossover iron(II) complexes based on dihydrobis(1-pyrazolyl)borate are presented here. The complexes {Fe[H(2)B(Pz)(2)](2)(phen)} (phen = 1,10-phenanthroline), 1, and {Fe[H(2)B(Pz)(2)](2)(bipy)} (bipy = 2,2'-bipyridine), 2, have been synthesized and their structures determined by X-ray diffraction methods. Crystals 1 and 2 are monoclinic, space group C2/c, Z = 4 with a = 17.448(4) Å, b = 16.101(4) Å, c = 10.611(2) Å, and beta = 112.47(2) degrees for 1 and a = 16.307(2) Å, b = 15.075(4) Å, c = 11.024(4) Å, and beta = 114.95(5) degrees for 2 at 293 K. The crystal structure of 2 was also determined at 139 K in order to detect the structural changes associated with the S = 0 <--> S = 2 spin conversion. 2 retains the same space group upon spin conversion with a = 16.086(6) Å, b = 14.855(6) Å, c = 10.812(2) Å, and beta = 114.18(3) degrees. The structures of 1 and 2 are made up of mononuclear neutral species where the positive charge of iron(II) is neutralized through the coordination of two chelate bidentate dihydrobis(pyrazolyl)borate anions, and phen or bipy neutral ligands are used to fill the iron(II) coordination sphere. The molecular structures for both compounds are very similar, with Fe-N bond lengths in the 2.212-2.158 Å range for the high-spin phase. The structural modifications associated with the spin change in 2 mainly consist of a large reorganization of the metal environment: the Fe-N decreases by 0.15 Å (mean value) when the temperature is lowered from 290 to 139 K and a more regular shape of the [FeN(6)] octahedron is achieved through a slight modification of the trigonal deformation angle from 5.3 degrees to 3.2 degrees along with remarkable variations of the N-Fe-N angles. The thermodynamic model of Slichter and Drickamer was applied to account for the magnetic data. The intermolecular interaction parameter and the enthapy and entropy changes associated with the spin transition were estimated as Gamma = 3.3 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 81.9 J mol(-)(1) K(-)(1) for 1 and Gamma = 1.7 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 83.9 J mol(-)(1) K(-)(1) for 2, respectively.

An alternating di-(μ-(end-on)azido)−di-(μ-(end-to-end)azido) manganese(II) one-dimensional compound, with formula [Mn(bipy)(N3)2] (bipy = 2,2‘-bipyridine), has been synthesized and characterized. Its crystal structure has been solved at room temperature. The complex crystallizes in the triclinic P1̄ space group, with a = 7.547(2) Å, b = 9.137(4) Å, c = 9.960(4) Å, α = 110.76(4)°, β = 104.43(2)°, γ = 100.41(3)°, and Z = 2. The structure consists of manganese chains in which the MnII ions are alternatively bridged by two end-on (EO) and two end-to-end (EE) azido bridges. Each MnII ion has an octahedral coordination, completed by the two nitrogen atoms of the bipy ligand. The EO and EE bridges are arranged cis. This constitutes the first example of such an azido bridge chain for any metallic ion. ESR measurements show signals corresponding to ΔMs = 1 and ΔMs = 2 transitions, with no significant variations by modifying the temperature. The thermal variation of molar susceptibility reveals the existence of alternating ferro- and antiferromagnetic interactions, through alternating EO and EE azido bridges, in the compound. A theoretical model has been developed for an S = 5/2 alternating ferromagnetic−antiferromagnetic coupled 1D system:  the exchange parameters obtained with this model, considering the spin Hamiltonian H = −J 1∑S 2 i S 2 i +1 − J 2∑S 2 i +1 S 2 i +2, are J 1 = 13.8 K, J 2 = −17.01 K with g fixed at 2.0. Extended Hückel calculations are discussed to model the end-on and end-to-end bridged systems.

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A group of P-stereogenic monodentate phosphines S-PPhRR′ (R ) 1-naphthyl, 9-phenanthryl, or o-biphenylyl and R′ ) CH 3 -, i-C 3 H 8 -, and Ph 3 SiCH 2 -) have been prepared by succesive substitution reactions on the oxazaphospholidineborane obtained from (-)ephedrine and bis(N,N-diethylamino)phenylphosphine. The reaction with binuclear allyl compounds [Pd(µ-Cl)(allyl)] 2 gives neutral [PdCl(allyl)P*] complexes. When allyl ) 2-CH 3 -C 3 H 4 (5), two isomers appeared in solution due to the R-or S-geometry around the palladium atom. The discrimination effect of the phosphines is small and the maximum isomeric ratio is observed for PPh(o-Ph 2 )(CH 2 SiPh 3 ). The molecular structure determined by X-ray diffraction of two complexes with P* ) PPh(o-Ph 2 )(i-Pr) and PPh(o-Ph 2 )(OMe) showed a very similar nonsymmetric coordination of the allyl moiety according to the greater trans influence of the phosphorus atom. When allyl ) 1-C 6 H 5 -C 3 H 4 (6), the NMR spectroscopy showed up to four isomers due to the R-or S-geometry around palladium and the Z-or E-disposition of P* and the phenyl substituent of the allyl moiety. The E-isomers are the major species in solution, unique with PPh(o-Ph 2 )(CH 2 SiPh 3 ). The usual, well-defined dynamic exchanges by π-σ-π and pseudorotation of the allyl moiety have been observed. The codimerization reaction between styrene and ethylene has been tested using filtered CH 2 Cl 2 solutions of [PdCl(2-CH 3 -C 3 H 4 )P*] (5) complexes and AgBF 4 as catalytic precursors. Moderate activity (TOF < 225 h -1 at 25 °C) and good selectivities to 3-Ph-1-butene (∼90% at 80% conversion) are obtained. The ee is moderate (<40% ee) and different from the discrimination effects observed in the solutions of neutral complexes [PdCl(ally)P*]. The reaction carried out with deuterated styrene shows the clean C-H addition to the vinyl double bond of stryrene and confirms the irreversible nature of the insertion of styrene in the palladium hydride intermediate. The hydrovinylation reaction using substituted styrene with a potentially secondary coordination atom occurs only when the substitution is in the phenyl ring and without significant improvements of the ee.

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Cu06 octahedron20 as well as the enhancement due to increased magnetic dipole transition probability. The latter observation supports our contention that the metal ions are exchange coupled.Acknowledgments. S.H.W. thanks Dr. George A. Candela of the National Bureau of Standards for lending his expertise in the magnetic susceptibility measurements, A.L.R. thanks NSF for supporting the purchase of the diffractometer, and G.F.K. acknowledges the donors of the Petroleum Research Fund, ad-ministered by the American Chemical Society, for partial support of this work.

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