Reaction of [Mo~(CO),(~-C,M~,)~] with methylenecyclopropane and AgBF, in CH2CI2 affords the cationic trimethylenemethane complex [Mo{q4-C(CH2) 3)( CO) 2(q-C5Me5)] [B F, ] . Methylenecyclopropane and 2,2-dimethylmethylenecyclopropane react with [Mo( NCMe),(CO),(q-C,H,)][ BF4][BF,] confirmed that ring opening of methylenecyclopropane had occurred. The complex crystallises in the centrosymmetric orthorhombic space group Pbca with a = 12.822(2), b = 12.31 1 (3), c = 22.660(4) A, and Z = 8 ion pairs. The structure has been solved by conventional methods and refined by fullmatrix least squares to R = 0.0674 for 2 747 observed reflections at 268 f 1 K. In the cation the trimethylenemethane ligand adopts an orientation that is syn with respect to the OC-Mo-CO angle. It is pyramidal with the CH2 groups bent towards the molybdenum atom by an average of 12.4'. There is intramolecular congestion involving the C,Me5 and C(CH2)3 ligands that may contribute towards the observed asymmetric bonding of the former to the metal atom. Extended Huckel molecular-orbital calculations suggest that the observed syn stereochemistry is electronically preferred, and that the barrier to rotation of the T ~, -C ( C H ~) ~ ligand relative to a Mo(CO),(q-C,H,)fragment is high. The stereochemistry of the ring-opening reaction is disrotatory-out as exemplified by the conversion of cisand trans-2,3-dimethylmet hylenecyclopropane into syn,syn-dimet hyl and syn,anti-dimet hyl-trimethylenemethane complexes. The reaction of these cationic q4-trimethylenemethane complexes with the nucleophiles BH4 -, OH-, CuMe,-, and SPh-affords q3-allylic complexes derived from attack on the peripheral carbons.to giveMethylenec>clopropanes react with compounds of Fe', Rh', Ir', Pt ", and Pt' to form simple q2-bonded alkene complexes in which the three-membered ring remains i n t a ~t . ~. ~ However, these small-ring compounds are also known to undergo transition-metal-mediated reactions where carbon-carbon cleavage occurs. This is illustrated by the reaction of [Fez-(CO),] with methylene-2-phenylcyciopropane forming tricarbonyl( q4-trimet hylenephenylmet hane)iron, a detailed study with deuterium-labelled methylene-2-phenylcyclopropane showing that a distotatory-out ring-opening reaction occurs. Furthermore, a recent study of the chloropalladation reactions of methylenecyclopropanes carrying alkyl substituents on the three-membered ring showed that cleavage of the 2.3-0 bond occurred in a disrotatory manner. Ring cleavage also occurs in the palladium(0)-catalysed cycloaddition reactions of methylenecyclopropanes, although the question as to whether these reactions involve q4-trimethylenemethane complexes remains to be answered. We had previously observed that [ M o ( C O ) ~( ~-C , M ~, ) ] + reacts with methylenecyclopropane to form the first cationic q4-bonded trimethylenemethane complex, and this paper describes a detailed study of this reaction.$ Prepared by protonation (HBF,-Et20) of [MoM~(CO)~(T&H~)] dissolved in CHzCIz followed by reaction with MeCN.'j
