OXYGEN ATOM TRANSFER REACTIONS OF NICKEL AND PALLADIUM NITRO COMPLEXES.

Abstract

The reactions of nitro complexes of nickel and palladium with CO have been examined to determine the mechanism(s) by which CO₂ is produced. The solution and solid state structures of square planar Ni(NO₂)₂(L)₂ reactants and pseudotetrahedral Ni(NO₂)(NO)(L)₂ products have been determined and related to their reactivity. Infrared, ³¹P{¹H}, and crystallographic data indicate rapid isomerization between nitro and nitrito bonding modes of the NO⁻₂ ligands. The crystal structures of Ni(NO₂)₂(PPh₂(Ch₂)₂PPh₂) (I), Ni(NO₂) (NO) (PMe₃)₂ (II), and [Ni(ONO) (NO) (PPh₂(CH₂)₂PPh₂]₂ (III), show the NO⁻₂ groups to be N-bonded in I and II and O-bonded in III. The nitrosyl ligands in II and III are non-linear (Ni-N-O = 165.5(8) ° and 153.4(8) °, respectively). Furthermore, III crystallizes as a dimer bridged by two phosphine ligands even though molecular weights show this complex to be monomeric in solution. Each Ni(NO₂) (NO) (L)₂ complex reacts with CO to produce stoichiometric amounts of Ni(NO₂) (NO) (L)₂ and CO₂. Rate date indicate the reaction proceeds associatively through formation of a carbonyl intermediate which has been directly observed in the reaction of Ni(NO₂)₂(P(C₆H₁₁)₃)₂ with CO. The reaction of C¹⁸O with Ni(NO₂)₂(PMe₃)₂ results in no incorporation of ¹⁸O into the nickel product while ¹⁸O is incorporated into CO₂ to form ¹⁸OC¹⁶O. The mechanism consistent with all of the data involves a rapid equilibrium between both forms of NO⁻₂ coordination followed by the reaction of CO with either isomer in the rate determining step to form a monocarbonyl complex. Irreversible oxygen atom transfer to CO and loss of CO₂ terminate the reaction. The corresponding square planar palladium complexes, Pd(NO₂)₂L₂, react with CO to form N₂O, CO₂ and novel tetranuclear palladium clusters (Pd₄(CO)₅L₄). A crystal structure of Pd₄(CO)₅ - (PMePh₂)₄ shows the cluster to be a distorted tetrahedron of metal atoms with one open edge and the five remaining edges each bridged by a carbonyl group.