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

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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.