The synthesis and characterization of [Co(T1Et4iPrIP)X2] {T1Et4iPrIP = tris(1‐ethyl‐4‐isopropylimidazolyl)phosphine; X = OTf– (1), Cl– (2), Br– (3)} and [Co(T1Et4iPrIP)(CH3CN)3](OTf)2 (4) were investigated. In compounds 1 and 4, T1Et4iPrIP binds in a tridentate fashion, whereas 2 and 3 feature bidentate ligation. Compound 1 additionally coordinates two triflate ligands to complete a distorted square pyramidal geometry. Compound 4 also binds three acetonitrile ligands to afford a distorted octahedral geometry. Compounds 2 and 3 exhibit a distorted tetrahedral geometry for which the ligands comprise two imidazole nitrogen atoms and two halides. Bidentate coordination by using a tris(imidazolyl)phosphine ligand is unprecedented. Compounds 2 and 3 can be derived from 1 by the addition of either Cl– or Br– salts, respectively, whereas 4 can be generated by dissolving 1 in acetonitrile. DFT studies indicate that the HOMO–LUMO gaps of all compounds are comparable; however, steric factors stabilize the formation of 2 and 3 over 1 in the presence of halide ligands.

In this work we report the synthesis of five new nickel(II) complexes all coordinated to the tripodal ligand tris(1-ethyl-4-i Pr-imidazolyl)phosphine (TlEt4iPrIP). They are [Ni(T1Et4iPrIP) and [Ni(T1Et4iPrIP)Cl 2 ] (5). The complexes serve as bioinorganic structural model complexes for histidine-coordinated nickel proteins. The X-ray structures have been determine for all complexes which feature coordination numbers 4-6. We investigated the spectroscopic interconversions for these compound in dichloromethane solution and demonstrate interconversion between 1-3 and conversion of 2 to 4. Complex 5 can be spectroscopically converted to the cation of 4 by dissolving it in dichloromethane. Fits of variable temperature magnetic susceptibility data yielded the following parameters: g = 1.944, D = −0.327 cm −1 , E/D = 3.706 for 1; g = 2.280, D = −0.365 cm −1 , E/D = 22.178 for 2; g = 2.000, D = −7.402 cm −1 , E/D = −0.272 for 3; g = 2.176, D = −0.128 cm −1 , E/D = −0.783 for 4; g = 2.258, D = 14.288 cm −1 , E/D = 0.095 for 5. DFT structure optimizations afforded HOMO and LUMO energies indicating that complex 1 is the most stable.