METHYLISOCYANIDE DERIVATIVES OF MOLYBDENOCENE AND TUNGSTENOCENE - PREPARATION, REACTIVITY AND ELECTRONIC-STRUCTURE - CRYSTAL-STRUCTURES OF [(ETA-5-C5H5)2WBR(CNME)]BR AND [(ETA-5-C5H5)2MOC(N(H)CH3)N(H)C(N(H)CH3)][BF4]2.CH3CN

The cationic complexes [Cp2MoH(CNMe)]I (1), [Cp2MoI(CNMe)]I (2), [Cp2Mo(CNMe)2)]X2 (3a, X = I; 3b, X = Cl; 3c, X = PF6) and [Cp2WBr(CNMe)]Br (4) are prepared from [Cp2MoHI] and [Cp2MX2]. The X-ray crystal structure of 4 shows normal end-on coordination of CNMe and no structural signs of significant metal-to-ligand back-donation. NaBH4 attack on 3c produces [CpMo(eta4-C5H6)(CNMe)2]PF6 (5) but CH3O-, PhS-, NEt3 and NH3 add to the CNMe ligands to give, respectively, [Cp2MoC(OCH3)N(CH3)CNCH3]PF6 (6), [Cp2Mo(CNCH3){C(SPh)N(H)CH3}][PF6]2 (7), [Cp2Mo(CNCH3){C(NEt3)NCH3}][PF6]2 (8) and [Cp2MoC{N(H)CH3}N(H)C{N(H)CH3}][PF6]2. CH3CN (9). Molecular orbital calculations of the extended Huckel type are used to interpret the regioselectivity of the nucleophilic attack and the bonding in the Fischer-type carbene complexes 6-9. The crystal structure of [Cp2MoC{N(H)CH3}N(H)C{N(H)CH3}][BF4]2.CH3CN, the BF4- salt corresponding to 9, is presented and the distortion observed in the metallacycle interpreted by EHMO calculations. The ylid complex [Cp2W(CNMe)(CHClPMe3)]Br2 (11) is formed by reaction of 4 with PMe3 in CH2Cl2. Deprotonation of 1 and Na/Hg reduction of 4 produce the d4 electron-rich complexes [Cp2MCNMe] (12, M = Mo; 13, M = W).