Low-temperature N-O bond cleavage of nitrogen monoxide in heterometallic carbonyl complexes. An experimental and theoretical study.

The reaction of Na[RuCp(CO) 2] with [MnCp'(CO) 2(NO)]BF 4 gives the corresponding heterometallic derivative [MnRuCpCp'(mu-CO) 2(CO)(NO)] (Cp = eta (5)-C 5H 5; Cp' = eta (5)-C 5H 4Me). In contrast, the group 6 metal carbonyl anions [MCp(CO) 2L] (-) (M = Mo, W; L = CO, P(OMe) 3, PPh 3) react with the Mn and Re complexes [M'Cp'(CO) 2(NO)]BF 4 to give the heterometallic derivatives [MM'CpCp'(mu-N)(CO) 3L] having a nitride ligand linearly bridging the metal centers (W-N = 1.81(3) A, N-Re = 1.97(3) A, W-N-Re = 179(1) (o), in [WReCpCp'(mu-N)(CO) 3{P(OMe) 3}]). Density-functional theory calculations on the reactions of [WCp(CO) 3] (-) and [RuCp(CO) 2] (-) with [MnCp(CO) 2(NO)] (+) revealed a comparable qualitative behavior. Thus, two similar and thermodynamically allowed reaction pathways were found in each case, one implying the displacement of CO from the cation and formation of a metal-metal bond, the other implying the cleavage of the N-O bond of the nitrosyl ligand and release of a carbonyl from the anion as CO 2. The second pathway is more exoergonic and is initiated through an orbitally controlled attack of the anion on the N atom of the NO ligand in the cation. In contrast, the first pathway is initiated through a charge-controlled attack of the anion to the C atom of a CO ligand in the cation. The CO 2-elimination pathway requires at the intermediate stages a close approach of the NO and CO ligands, which is more difficult for the Ru compound because of its lower coordination number (compared to W). This effect, when combined with a stronger stabilization of the initial intermediate in the Ru reaction, makes the CO 2-elimination pathway slower in that case.