Paramagnetic Resonance Studies of Bonding in [CrCl5(H2O)]2—

Abstract

Single‐crystal, room‐temperature electron spin resonance spectra are reported for the complex ion [CrCl₅(H₂O)]^2— in the host lattice (NH₄)₂[InCl₅(H₂O)]. The spectra are analyzed by means of a rhombic spin Hamiltonian and the spin‐Hamiltonian parameters are related to an LCAO‐MO description of the ligand—Cr³⁺ bonding. The complexity of the d³ system and the lack of sufficient experimental parameters prevented the direct evaluation of molecular orbital mixing coefficients from the ESR and optical data as was accomplished previously for tetragonal d¹ complexes. However, an independent estimate of the spin—orbit coupling is used to establish interrelationships among the four molecular orbital mixing coefficients of interest and serves as a basis for discussing the bonding in [CrCl₅(H₂O)]^2—. Evidence is found for pi bonding to the H₂O. Also, as in the case of the d¹ complexes, charge‐transfer states are found to contribute significantly to g and to the zero‐field splitting parameter D. The sign of their contribution to D is controlled by the mixing coefficients and can be either positive or negative; the contribution of doublet states appears to be small.