Magnetic exchange interactions between hydrated complexes of iron group ions: Mn2+, Co2+, and Ni2+ in La2Zn3(NO3)12⋅24H2O

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Abstract
The double nitrate crystals La2Zn3(NO3)12⋅24H2O have two sites, X and Y, into which the divalent iron group ions may be substituted. Significant exchange interactions exist for nearest neighbor X–X pairs and X–Y pairs. For Mn, Co, and Ni, there are six possible X–X pairs and nine possible X–Y pairs since the latter do not possess inversion symmetry. Electron paramagnetic resonance of pairs is used to study Mn(X) –Co(X), Ni(X) –Co(X), Ni(X) –Mn(Y), Co(X) –Mn(Y), and Mn(X) –Co(Y) interactions. These new results combined with earlier results for nine of the other ten possible combinations are used to analyze the interaction in terms of exchange between orbitals. For the X–Y interactions, it appears that only the eg orbitals are significantly involved, and a model is developed for a semiquantitative explanation of that interaction in terms of the covalency of the complex and the two hydrogen bonds between the water molecules of the X and Y complexes and a noncomplexed water molecule which is a receptor for the bonds. The degree of covalency required in the hydrogen bonds appears to be in accord with current descriptions. For X–X pairs it is clear that t2g orbitals are involved in the exchange process, and an attempt is made to use the data for X–X pairs to deduce the individual orbital contributions, assuming that the orbital exchange parameters do not vary with the number of d electrons. It is concluded that the strain induced by Mn ions in the lattice affects the orbital exchange parameters for the X–X pairs as it does for the X–Y pairs, and this prevents a definitive determination. Values are deduced for three of the seven orbital exchange constants which are valid if none of the other four are exceptionally large.