<p>To date, heterometal-incorporated Keggin clusters, as an important branch of polyoxometalate chemistry, have remained limited to Nb^V/Mo^VI/W^VI-based Keggin architectures. In this work, we successfully designed and synthesized the first such structure based on p-block elements, taking into account spatial compatibility and the presence of an inactive shell. The resulting <b>CTGU-SnC-19</b> cluster is a unique sandwich-type structure, featuring a central electron-donating {Bi₂(NO₃)₄}²⁺ moiety flanked by electron-accepting {<i>γ</i>-BiSn₁₁}⁺ units on both sides. It is also the largest tin-oxo structure with a heterometal cluster reported to date. Theoretical calculations indicate that each component of this sandwich-type structure works synergistically to promote efficient bidirectional electron transfer from the center toward both ends, significantly enhancing its third-order nonlinear optical property. Moreover, the excellent optical limiting performance of <b>CTGU-SnC-19</b> was experimentally demonstrated through the Z-scan measurements. This work provides new insights into the construction and property modulation of heterometal-incorporated Keggin clusters.</p>