REACTIONS OF BARE AND LIGATED CHROMIUM(I) IONS WITH GASEOUS ARENES - ROLE OF A SPECTATOR AROMATIC RING IN CHELATE COMPLEX-FORMATION

The gas-phase reaction of XCr(+) (X = C6H6, NH3) and (NH3Cr+ with aren es (L) leads to XCrL(+), LCr(+), and L(2)Cr(+) complexes. The pattern and the efficiency of the two classes of complex formation processes a llow a clear distinction between simple arenes, such as toluene and me sitylene, yielding ultimately sandwich-type L(2)Cr(+) complexes, and a lpha,omega-diphenylalkanes, yielding only LCr(+) complexes, where a co mmon sandwich-type structure is achieved by simultaneous coordination of the metal cation with the two aryl rings. The critical parameter co ntrolling the extent of chelate coordination is the length of the meth ylene chain joining the two rings. A different behavior is displayed b y ligands such as trans-stilbene and [2.2]paracyclophane, where only o ne aryl ring is involved in the stepwise NH3 substitution from (NH3Cr. Under low pressure, and hence inefficient collisional deactivation, the Crf reaction with the selected arenes is governed by relative bind ing energies and radiative cooling efficiencies of the ensuing complex es. Insertion of free Cr+ into the cage of [2.2]paracyclophane may not be excluded.