(Cyclopentadienyl)nitrosylmanganese Compounds: The Original Molecules Containing Bridging Nitrosyl Groups

The (cyclopentadienyl)nitrosylmanganese derivatives Cp 2 Mn 2 (NO) 2 (CO) n ( n = 2, 1, 0), Cp 2 Mn 2 (NO) 3 X (X = NO 2 , ‐C 5 H 5 ), CpMn(NO) 2 , Cp 2 Mn 2 (NO) 4 , and Cp 3 Mn 3 (NO) 4 have been investigated by using density functional theory. Six structures are found for Cp 2 Mn 2 (NO) 2 (CO) 2 with various combinations of terminal and bridging carbonyl and nitrosyl groups within ca. 3 kcal/mol of each other indicating a highly fluxional system. A triplet‐state triply bridged structure is predicted to be the global minimum for Cp 2 Mn 2 (NO) 2 (CO), just as it is for the isoelectronic Cp 2 Fe 2 (CO) 3 . For Cp 2 Mn 2 (NO) 2 a doubly bridged triplet structure is predicted to lie ca. 30 kcal/mol below the corresponding singlet structure, unlike the isoelectronic Cp 2 Fe 2 (CO) 2 . The structures predicted for Cp 2 Mn(µ‐NO) 2 (NO)X (X = NO 2 and η 1 ‐C 5 H 5 ) and Cp 3 Mn 3 (µ‐NO) 3 (µ 3 ‐NO) are close to the experimentally known structures. The monomer CpMn(NO) 2 , with a favorable 18‐electron configuration for the manganese atom, is predicted to be unstable with respect to dimerization to Cp 2 Mn 2 (µ‐NO) 2 (NO) 2 , for which two structures are found of similar energy. One Cp 2 Mn 2 (µ‐NO) 2 (NO) 2 structure has a non‐bonding Mn ··· Mn distance of ca. 3.1 Å and linear terminal MnNO units, whereas the other Cp 2 Mn 2 (µ‐NO) 2 (NO) 2 structure has a single‐bond Mn–Mn distance of ca. 2.5 Å and bent terminal MnNO units. In Cp 2 Mn(µ‐NO) 2 (NO)(η 1 ‐Cp) the Mn 2 (µ‐NO) 2 unit is unsymmetrical, with significantly different (by ca. 0.2 Å) Mn–N bond lengths to each side. However, in Cp 2 Mn 2 (NO) 2 (CO) n ( n = 2, 1, 0) and Cp 2 Mn 2 (µ‐NO) 2 (NO) 2 the Mn 2 (µ‐NO) 2 units are symmetrical, with essentially the same Mn–N bond lengths to both sides. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)