Coordination complexes of lanthanide metals with tris-1-naphthylphosphine oxide (Nap
3PO, L) have not been previously reported in the literature. We describe here the formation of lanthanide(III) nitrate complexes Ln(NO
3)
3L
4 (Ln = Eu to Lu) and the structures
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Coordination complexes of lanthanide metals with tris-1-naphthylphosphine oxide (Nap
3PO, L) have not been previously reported in the literature. We describe here the formation of lanthanide(III) nitrate complexes Ln(NO
3)
3L
4 (Ln = Eu to Lu) and the structures of [Ln(NO
3)
3L
2]·2L (Ln = Eu, Dy, Ho, Er) and L. The core structure of the complexes is an eight-coordinate [Ln(NO
3)
3L
2] with the third and fourth ligands H-bonded via their oxygen atoms to one of the naphthyl rings. The structures are compared with those of the analogous complexes of triphenylphosphine oxide and show that the Ln-O(P) bond in the Nap
3PO complexes is slightly longer than expected on the basis of differences in coordination numbers. The reaction solutions, investigated by
31P and
13C NMR spectroscopy in CD
3CN, show that coordination of L occurs across the lanthanide series, even though complexes can only be isolated from Eu onwards. Analysis of the
31P NMR paramagnetic shifts shows that there is a break in the solution structures with a difference between the lighter lanthanides (La–Eu) and heavier metals (Tb–Lu) which implies a minor difference in structures. The isolated complexes are very poorly soluble, but in CDCl
3, NMR measurements show dissociation into [Ln(NO
3)
3L
2] and 2L occurs.
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