Isoelectronic

{Sm}^{2 +}

and

{Eu}^{3 +}

ions with their ground configuration 4

f^{6}

are used in different ternary MYX compounds to study the reduction of the Slater parameters

F_{k}

and the spin-orbit coupling parameter

ζ_{4 f}

, commonly referred to as the nephelauxetic effect. At first, a simulation of the 4

f^{6}

energy levels in the intermediate coupling scheme with the two parameters

F_{2}

and

ζ_{4 f}

shows that the set of basis functions must include all the septets

(^{7}

L_{J}

) and all the quintets

(^{5}

L_{J}

), as well as some triplets

(^{3}

P_{J}

{}^{3}

D_{J}

, and

{}^{3}

F_{J}

) or singlets

(^{1}

S_{J}

{}^{1}

P_{J}

, and

{}^{1}

D_{J}

) to reproduce the energy level scheme accurately, whereby a strong effect of the triplets and singlets on the

{}^{5}

D_{J}

multiplets is most prominent. With structural parameters of different host materials at ambient and elevated pressures, different distance dependences of the parameters

F_{2}

and

ζ_{4 f}

for

{Sm}^{2 +}

and

{Eu}^{3 +}

are derived from the ambient-pressure and high-pressure energy-level schemes, demonstrating a breakdown of the nephelauxetic series. Only a combination of at least two microscopic covalency mechanisms, central-field covalency (CFC) and symmetry-restricted covalency (SRC), allows for a description without contradictions. Thereby the CFC contribution is dominant for the small

{Eu}^{3 +}

ion, whereas the SRC is more dominant for the larger

{Sm}^{2 +}

ion. Finally, ab initio calculations of Coulomb integrals are used to evaluate the Slater parameters

F_{k}

{Sm}^{2 +}

within the SRC model and close agreement of these results with the experimental data is then obtained.

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