Diolefin dimerization catalyst and method for producing nitrosyl halides of iron triad metals

A catalyst system formed by contacting components consisting essentially of (1) at least one elemental metal selected from the group consisting of manganese, tin, and zinc with (2) at least one nitrosyl metal halide selected from the group consisting of nitrosyl metal halides having the formulas [Fe(NO).sub.2 X].sub.y, [Co(NO).sub.2 X].sub.y, [Ni(NO)X].sub.y, Fe(NO).sub.2 (L)X, Ni(NO)(L)X, and Co(NO).sub.2 (L)X, wherein X is selected from the group consisting of chloride, bromide, and iodide, y is 1 or 2 for Co and Fe and 1, 2, 3, or 4 for Ni, and wherein (L) is selected from the group of compounds having the formulas

• R.sub.3 M, (RO).sub.3 M, SR', R--S--R, R.sub.3 MO, OR', and R--O--R
• wherein each R is individually selected from the group consisting of hydrocarbyl aromatic radicals, hydrocarbyl aliphatic radicals, halo-substituted hydrocarbyl aromatic radicals, halo-substituted aliphatic hydrocarbyl radicals, alkoxy-substituted hydrocarbyl aromatic radicals and alkoxy-substituted aliphatic hydrocarbyl radicals having up to about 20 carbon atoms, wherein R' is a divalent saturated or olefinically unsaturated hydrocarbyl radical having 3 to 7 carbon atoms, and wherein M is phosphorus, antimony, or arsenic.