The Ligand Field of the Azido Ligand: Insights into Bonding Parameters and Magnetic Anisotropy in a Co(II)–Azido Complex

The azido ligand
is one of the most investigated ligands in magnetochemistry.
Despite its importance, not much is known about the ligand field of
the azido ligand and its influence on magnetic anisotropy. Here we
present the electronic structure of a novel five-coordinate Co­(II)–azido
complex (1), which has been characterized experimentally
(magnetically and by electronic d–d absorption spectroscopy)
and theoretically (by means of multireference electronic structure
methods). Static and dynamic magnetic data on 1 have
been collected, and the latter demonstrate slow relaxation of the
magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static
susceptibility and magnetizations (D = −10.7
cm–1, E/D = 0.22)
are in excellent agreement with the value of D inferred
from an Arrhenius plot of the magnetic relaxation time versus the
temperature. Application of the so-called N-electron
valence second-order perturbation theory (NEVPT2) resulted in excellent
agreement between experimental and computed energies of low-lying
d–d transitions. Calculations were performed on 1 and a related four-coordinate Co­(II)–azido complex lacking
a fifth axial ligand (2). On the basis of these results
and contrary to previous suggestions, the N3– ligand is shown to behave as a strong σ and π donor.
Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co–N
bond distances) of the axial ligand on the N3– site. The effect on the change in sign of D in
going from four-coordinate Co­(II) (positive D) to
five-coordinate Co­(II) (negative D) is discussed
in the light of the bonding scheme derived from ligand field analysis
of the ab initio results.