An Easy Entry to Dimers [{RuX(μ-X)(CO)(P⌒P)} 2 ] (X = Cl, Br; P⌒P = 1,1‘-Bis(diphenylphosphino)ferrocene, 1,1‘-Bis(diisopropylphosphino)ferrocene) from η 3 -Allylruthenium(II) Derivatives [RuX(η 3 -2-C 3 H 4 R)(CO)(P⌒P)] (R = H, Me):  Efficient Catalyst Precursors in Transfer Hydrogenation of Ketones §