SUBSTITUTION AND ADDITION-REACTIONS OF THE (CO)3(ETA-7-TROPYLIUM)M CATIONS (M=CR, MO, W) WITH TERTIARY DISPHOSPHINES

The reactions of the title compounds I-III, [(eta7-C7H7)M(CO)3]BF4 (M = Cr, Mo, and W, respectively), with the diphosphines PPh2(CH2)nPPh2 (n = 1-4) at low temperatures (-40 to -60-degrees-C) form two new series of complexes which are quite different from previous carbonyl substitution products. The first series (IV-VI) formed by the diphosphines (n = 1-3) comprise 7-exo ring adducts involving the bonding of only one phosphorus atom of the diphosphine to ring carbon atom 7 of I. The second series (VII and VIII) are diphosphine ring linked dimers, e.g. [(7,7'-exo-dppb){eta6-C7H7)Cr(CO)3}2][BF4]2, for which X-ray crystallography confirms a diphosphine linked ring-ring structure with the Cr(CO)3 moieties situated on opposite sides of the ring systems. The ring C-phosphorus bond length of 1.852 angstrom indicates a relatively weak bond, consistent with facile cleavage of these adducts on reaction with nucleophiles such as hydride.