Heptacarbonyldiosmium and Hexacarbonyldiosmium: Two Highly Unsaturated Binuclear Osmium Carbonyls*

A total of nine singlet structures for Os2(CO)7 and 15 structures (nine singlet and six triplet) for Os2(CO)6 have been found by density functional theory thereby indicating very complicated energy surfaces. The global minimum for Os2(CO)7 is a doubly carbonyl bridged structure Os2(CO)5(μ-CO)2 with an Os=Os distance of 2.67 Å suggesting a formal double bond and hence a 16-electron rather than an 18electron configuration for one of the osmium atoms. However, at only slightly higher energy (3.2 kcal mol) lies an unbridged Os2(CO)7 structure with a shorter OsOs distance of 2.54 Å, corresponding to a formal triple bond and an 18-electron configuration for each osmium atom. The global minimum for Os2(CO)6 can be derived from that of Os2(CO)7 by removal of a carbonyl group while retaining the Os=Os double bond and the two bridging carbonyl groups. Slightly higher energy Os2(CO)6 structures at ≈3 kcal mol or more above the global minimum have short Os Os  distances around 2.4 Å, consistent with the formal quadruple bonds necessary to give both osmium atoms the favored 18-electron configuration. None of the 24 structures for Os2(CO)7 and Os2(CO)6 found in this work has a four-electron donor -μCO bridging carbonyl group.