You just chose the right answer. Dividing the carbonyls among the osmium atomes gives 4 carbonyls per osmium. With the 8 valence electrons from metal, 4 electrons from the 2 terminal carbonyls and 4 electrons from the 4 bridging carbonyls this brings the total to 16 electrons. The only remaining place to get electrons is from metal-metal bonds. Therefor, each osmium needs 2 Os-Os bonds. The only possible shape where each osmium is bonded to 2 other osmiums is a triangle.

Os            = 8e- 2 x COT   = 4e- 4 x COB   = 4e- 2 x Os-Os = 2e-                    18e-

But there is another alternative structure, that also obeys The Eighteen Electron Rule. You can not predict which of the possible structures will be formed. You would need to use Infrared Spectroscopy to indentify which structure has been made.

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