A metallostibine, Os2(CO)8(µ-SbPh) (2), was generated readily and in high yield and purity from the Os3(CO)11(µ-SbPh)(Cl)2 (1) cluster through elimination of the terminal Os(CO)3(Cl)2 Lewis acid fragment with activated γ-alumina. Compound 2 is air sensitive in solution but relatively stable as a solid. The Sb atom in 2 adopts a trigonal pyramidal geometry in both the solid and solution states, with an active lone pair of electrons. Experimental and computational studies show that 2 is a strong two-electron donor with a donor strength comparable to that of alkylphosphines. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim