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The complexes [Fe[HC(3,5-Me2pz)3]2](BF4)2 (1), [Fe[HC(pz)3]2](BF4)2 (2), and [Fe[PhC(pz)2(py)]2](BF4)2 (3) (pz = 1-pyrazolyl ring, py = pyridyl ring) have been synthesized by the reaction of the appropriate ligand with Fe(BF4)2.6H2O. Complex 1 is high-spin in the solid state and in solution at 298 K. In the solid phase, it undergoes a decrease in magnetic moment at lower temperatures, changing at ca. 206 K to a mixture of high-spin and low-spin forms, a spin-state mixture that does not change upon subsequent cooling to 5 K. Crystallographically, there is only one iron(II) site in the ambient-temperature solid-state structure, a structure that clearly shows the complex is high-spin. Mössbauer spectral studies show conclusively that the magnetic moment change observed at lower temperatures arises from the complex changing from a high-spin state at higher temperatures to a 50:50 mixture of high-spin and low-spin states at lower temperatures. Complexes 2 and 3 are low-spin in the solid phase at room temperature. Complex 2 in the solid phase gradually changes over to the high-spin state upon heating above 295 K and is completely high-spin at ca. 470 K. In solution, variable-temperature 1H NMR spectra of 2 show both high-spin and low-spin forms are present, with the percentage of the paramagnetic form increasing as the temperature increases. Complex 3 is low-spin at all temperatures studied in both the solid phase and solution. An X-ray absorption spectral study has been undertaken to investigate the electronic spin states of [Fe[HC(3,5-Me2pz)3]2](BF4)2 and [Fe[HC(pz)3]2](BF4)2. Crystallographic information: 2 is monoclinic, P2(1)/n, a = 10.1891(2) A, b = 7.6223(2) A, c = 17.2411(4) A, beta = 100.7733(12) degrees, Z = 2; 3 is triclinic, P1, a = 12.4769(2) A, b = 12.7449(2) A, c = 13.0215(2) A, alpha = 83.0105(8) degrees, beta = 84.5554(7) degrees, gamma = 62.5797(2) degrees, Z = 2.

The reaction of Pb(acac)2 with 2 equiv of [H(Et2O)2][B[3,5-(CF3)2C6H3]4] (HBAr(f)) in CH2Cl2 followed by addition of 2 equiv of either HC(pz)3 or HC(3,5-Me2pz)3 (pz = pyrazolyl ring) leads to the formation of [Pb[HC(pz)3]2][B[3,5-(CF3)2C6H3]4]2 (1) and [Pb[HC(3,5-Me2pz)3]2][B[3,5-(CF3)2C6H3]4]2 (2), respectively. The cation in 1 has a distorted octahedral structure with a stereochemically active lone pair on lead(II). In contrast, the cation in 2 is trigonally distorted octahedral with the lone pair on the lead(II) clearly stereochemically inactive. The driving force for this cation to have a stereochemically inactive lone pair is that in this geometric arrangement the interligand distances between adjacent 3-position methyl groups are close to 4.0 A, the sum of the van der Waals radii of two methyl groups. To facilitate this chemistry, the synthesis of Na[B[3,5-(CF3)2C6H3]4], needed to prepare HBAr(f), has been dramatically improved. The main change is to add NaBF4 to the reaction mixture before forming the Grignard from the reaction of magnesium and 3,5-(CF3)2C6H3Br. The Grignard reacts with the NaBF4 as it forms, reducing the danger of explosion and leading to a higher isolated yield of the product. Crystallographic information: 1 is triclinic, P1, a = 13.0133(6) A, b = 17.2210(7) A, c = 24.7634(11) A, alpha = 71.7300(10) degrees, beta = 82.3630(10) degrees, gamma = 70.5120(10) degrees, Z = 2; 2 is triclinic, P1, a = 12.756(4) A, b = 13.469(4) A, c = 17.160(5) A, alpha = 82.454(7) degrees, beta = 89.904(8) degrees, gamma = 72.995(7) degrees, Z = 1.

The complex [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (pz = pyrazolyl ring) undergoes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this phase was performed at 220 K yielding the cell constants a = 20.338(2) A, b = 10.332(1) A, c = 19.644(2) A, beta = 111.097(2) degrees, and V = 3851.5(6) A(3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS and LS. The radical change in the coordination sphere for half of the iron(II) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 A, that accompanies this magnetic transition causes a phase transition. The crystal system changes from C-centered monoclinic to primitive triclinic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P1 with a = 10.287(2) A, b = 11.355(3) A, c = 18.949(4) A, alpha = 90.852(4) degrees, beta = 105.245(4) degrees, gamma = 116.304(4) degrees, and V = 1892.3(8) A(3). All specimens investigated below the phase transition temperature were determined to be nonmerohedral twins. Temperature cycling between these two forms does not appear to degrade crystal quality. Previous magnetic susceptibility measurements indicate a second, irreversible increase in the magnetic moment the first time the crystals are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not cooled below 220 K.

The reaction of M(BF(4))(2).xH(2)O (M = Co, Ni, and Cu) and HC(3,5-Me(2)pz)(3) in a 1:2 ratio yields [Co[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (2), [Ni[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (3), and [Cu[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (4). Over the temperature range from 5 to 350, 345, or 320 K, Curie law behavior is observed for microcrystalline samples of all three compounds showing them to have three, two, and one unpaired electrons, respectively, with no spin-crossover observed for 2. Crystalline samples of these compounds torque in the applied magnetic field the first time the sample is cooled to 5 K. The solid-state structures of all three are isomorphous at 220 K, monoclinic in the space group C2/c. The metal is located on a unique crystallographic site and has a trigonally distorted octahedral structure, with 4 showing the expected Jahn-Teller distortions. Cooling crystals of all three to low temperatures leads to the observation of the same phase change to triclinic in the new space group P(-)1 with nonmerohedral twinning. This change is reversible and yields two crystallographically unique metal sites at low temperature. The bond angles and distances for the two different metal sites for each compound in the low temperature structures are very similar to each other and to those in the 220 K structures. The same phase change, monoclinic to triclinic, has been observed previously for [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (1), except in this case, the phase change results in half of the cations changing over from the high-spin state to the low-spin state while the other half of the cations remain high-spin, with the low-spin form decreasing its Fe-N bond distances by 0.19 A. The new results with 2-4 show that it is the phase transition, which occurs in complexes of the type [M[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) with first row transition metals, that is driving the unusual spin-crossover behavior of [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2).