Electronic Structures of Mixed-Sandwich Complexes of Cyclopentadienyl and Hydrotris(pyrazolyl)borate Ligands with 3d Transition Metals

• Published in: Inorganic chemistry 2003-07, Vol.42 (14), p.4366-4381
• Main Authors: Brunker, Tim J, Green, Jennifer C, O'Hare, Dermot
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic030073u

The electronic and magnetic properties of a series of mixed-sandwich complexes MCpRTp (CpR = Cp or Cp*; Tp = hydrotris(pyrazolyl)borate; M = V, Cr, Fe, Co or Ni) have been studied and compared to their homoleptic analogues, MCpR 2 and MTp2. Solid-state magnetic susceptibility measurements and EPR spectroscopic data indicate that complexes with d3, d6, and d8 configurations are similar electronically to their metallocene analogues, except for FeCp*Tp, which displays a spin equilibrium (S = 0 ⇔ S = 2) in solution which was investigated by variable- temperature NMR spectroscopy. The d2 complex [VCpTp]+ displays magnetic behavior consistent with an orbitally nondegenerate ground state. The d4 species CrCp*Tp has a high-spin (S = 2) ground state. The d7 species CoCpTp is high spin (S = 3/2) whereas its Cp* analogue and [NiCp*Tp]+ are both low-spin (S = 1/2) species. The optical spectra of the d3, d6, and d8 complexes were assigned in a fashion similar to the analogous metallocenes and ligand-field parameters (Δ1 = δ-σ gap, Δ2 = σ-π gap for d-orbitals in axial symmetry) calculated. The analysis shows that for 15-electron species the total ligand-field splitting, ΔTOT, is larger than for their metallocene analogues, whereas for the 18-electron case ΔTOT is smaller and for 20-electron systems ΔTOT is approximately the same. In all cases Δ2 is substantially reduced compared to the metallocenes, and in the majority of cases Δ1 is markedly larger. DFT calculations were performed to investigate further the nature of the ligand environment on the frontier orbitals in these complexes. Orbital energies and compositions were calculated and compared for a series of homoleptic and mixed-sandwich complexes of Ni(II) and across the 1st transition series for MCpRTp species. The ability of Tp (vs Cp) to act as a δ-donor (with respect to the principal molecular axis) imparts significant ligand antibonding character to the δ-orbitals and results in decreased επ-εδ values compared to the metallocenes and an increased tendency toward high-spin complexes in the mixed-sandwich complexes. Structure calculations were performed for CrCp*Tp, [VCpTp]+, and CoCp*Tp which show substantial distortions from ideal axial symmetry in their crystal structures. The origins of these distortions were confirmed to arise from unequal occupancy of near-degenerate δ- and π-levels.