MOSSBAUER-SPECTROSCOPY OF SPIN-COUPLED IRON-CHROMIUM COMPLEXES - MU-HYDROXO-BIS(MU-ACETATO)-BRIDGED IRON(2-CHROMIUM(3+) AND MU-OXO-BIS(MU-ACETATO)-BRIDGED IRON(3+)-CHROMIUM(3+)())

We have analyzed Mossbauer spectra of a model complex of known structu re with an Fe2+(S-1=2)-mu OH-Cr3+(S-2=3/2) center (A) and of its Fe3+( S-1=5/2)-mu O-Cr3+(S-2=3/2) analog (B). These mu-hydroxo and mu-oxo br idged binuclear metal centers display unusual magnetic properties as f ound in several diiron-oxo proteins, Our results confirm antiferromagn etic spin coupling between Fe and Cr ions which results in S-eff = 1/2 and S-eff = 1 ground states for A and B, respectively. The isotropic exchange H-ex = JS(1) . S-2 is weaker for the mu-hydroxo (J approximat e to 21 cm(-1)) than for the mu-oxo (J approximate to 275 cm(-1)) comp lex. Spectra recorded at 4.2 K, in fields of 0.22-4.7 T, have been ana lyzed with the effective spin Hamiltonian for the ground state H-eff = beta S-eff.(g) over tilde(eff). H + S-eff.(A) over tilde(1)(eff). I-1 + I-1 .(P) over tilde(1) . I-1 - beta(n)g(n)H . I-1. For complex B, t he zero-field splitting S-eff. D-eff. S-eff is also included in R(eff) . In applied fields, the 4.2 K spectra of Fe2+ in A showed hyperfine s plittings which allowed the determination of the following S-eff = 1/2 Hamiltonian parameters: 1/3 Tr (g) over tilde(eff) approximate to 2.0 0, (A) over tilde(1)(eff)/g(n) beta(n) = -(18.3,5.6,25.0) T, Delta E(Q ) = +2.87 mm/s, eta = 0.93, and delta(Fe) = 1.22 mm/s. The weak coupli ng of A allows the zero-field splitting to mix higher spin manifolds w ith the ground state doublet, and, to obtain intrinsic parameters, we also calculated the spectra of Fe2+ by diagonalizing the (2S(1) + 1 = 5) x (2S(2) + 1 = 4) matrix of the Hamiltonian H = JS(1) . S-2 + Sigma (i=1)(2){S-i .(D) over tilde(i) . S-i + beta S-i .(g) over tilde(i) . H} + S-1 .(a) over tilde(1) . I-1 +I-1 + I-1 .(P) over tilde(1) . I-1 - beta(n)g(n)H . I-1. We determined the following parameters for Fe2+: D-1 = +4.0 cm(-1), E(1) = +0.4 cm(-1), 1/3 Tr (g) over tilde(1) great er than or similar to 2.07, <(alpha)over tilde>(1)/g(n) beta(n) = -(10 .2,3.5,15.6) T. For complex B, we found that Fe3+ has a large quadrupo le splitting (Delta E(Q) = -2.00 mm/s, eta = 0.22) presumably as a res ult of anisotropic covalency due to the close proximity of the bridgin g O2-. This large Delta E(Q) is comparable to values found in diiron-o xo proteins. Spectra of B in applied fields also displayed hyperfine s plittings, and the following S-eff = 1 Hamiltonian parameters could be deduced: D-eff = +3.9 cm(-1), E(eff) = +1.7 cm(-1), 1/3 Tr (g) over t ilde(eff) = 2.01, (A) over tilde(1)(eff)/g(n) beta(n) = -(33.8,30.9,35 .8) T, delta(Fe) = 0.52 mm/s.