The luminal A-subtype of breast cancer, where the oestrogen receptor α (ERα) is overexpressed, is the most frequent one. The prodrug tamoxifen (
1) is the clinically used agent, inhibiting the ERα activity via the formation of several active metabolites, such as
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The luminal A-subtype of breast cancer, where the oestrogen receptor α (ERα) is overexpressed, is the most frequent one. The prodrug tamoxifen (
1) is the clinically used agent, inhibiting the ERα activity via the formation of several active metabolites, such as 4-hydroxytamoxifen (
2) or 4,4′-dihydroxytamoxifen (
3). In this study, we present the tamoxifen derivative 4-[1,1-bis(4-methoxyphenyl)but-1-en-2-yl]-2,2′-bipyridine (
4), which was combined with platinum or palladium dichloride, the former a well-known scaffold in anticancer treatment, to give [PtCl
2(
4-κ
2N,
N′)] (
5) or [PdCl
2(
4-κ
2N,
N′] (
6). To prevent fast exchange of weakly coordinating chlorido ligands in aqueous solution, a bulky, highly stable and hydrophobic
nido-carborate(−2) ([C
2B
9H
11]
2−) was incorporated. The resulting complexes [3-(
4-κ
2N,
N′)-3,1,2-PtC
2B
9H
11] (
7) and [3-(
4-κ
2N,
N′)-3,1,2-PdC
2B
9H
11] (
8) exhibit a dramatic change in electronic and biological properties compared to
5 and
6. Thus,
8 is highly selective for triple-negative MDA-MB-231 cells (IC
50 = 3.7 μM, MTT test), while
7 is completely inactive against this cell line. The observed cytotoxicity of compounds
4–
6 and
8 against this triple-negative cell line suggests off-target mechanisms rather than only ERα inhibition, for which these compounds were originally designed. Spectroscopic properties and electronic structures of the metal complexes were investigated for possible explanations of the biological activities.
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