The half-sandwich methylimido complexes [Nb(q-C,R,)(NMe) CI,] (R = H l a , or M e l b ) have been prepared by treatment of [Nb(q-C,R,) CI,] with NMe(SiMe,), in acetonitrile. The sterically hindered alkyland aryl-imido analogues [Nb(q-C,H,)(NR)CI,] (R = But lc, or C6H,Pri2-2,6 Id), [Nb(q-C,Me,)-(N-C6H,Pri,-2,6)CI,] l e and the tantalum compound [Ta(q-C,Me,)(N-C,H,Pri,-2,6)C12] 1 f are obtained by treatment of [M(q-C,R,)CI,] with t w o equivalents of NHR(SiMe,) in chlorocarbon solvent. Crystal structures of l a , lc, I d and I f show that these complexes are mononuclear with quasi-linear imido ligands. The metal-nitrogen bond distances range from 1.744(3) t o 1.780(5) A consistent with pseudo triple bonds. Compounds la-le react with tertiary phosphines t o give eighteen-electron adducts of the type [Nb(q-C,H,)(NR)CI,(PR',)] 2a-2e. The crystal structure of [Nb(q-C,H,)(NMe)CI,(PMe,)]shows a distorted four-legged piano-stool geometry in which the phosphine binds cis to the methylimido and cis t o one of the chloride ligands. Alkoxide and aryloxide derivatives of the type [Nb(q-C,R,) (NR')(OR"),] may be prepared via reactions of the dichlorides 1 with 2 equivalents of LiOR" in Et,O. Fenske-Hall quantumchemical calculations indicate that there exists a close relationship between the Nb(q-C,H,) (NR) fragment and the bent metallocene moiety M(T~-C,H,)~ for the Group 4 elements.

RhCl(CO)(PEt 3 ) 2 ] catalyses the carbonylation of methanol in the presence of MeI and water at a rate 1.8 times that for [RhI 2 (CO) 2 ] Ϫ at 150 ЊC. The reaction is first order in [MeI] and zero order in p CO . However, the phosphine complex degrades to [Rh(CO) 2 I 2 ] Ϫ during the course of the reaction. Stoichiometric studies show that the rate of oxidative addition of MeI to [RhI(CO)(PEt 3 ) 2 ] is 57 times faster than to [RhI 2 (CO) 2 ] Ϫ at 298 K and that [RhMeI 2 (CO)(PEt 3 ) 2 ] can be isolated and crystallographically characterised. Combination of the methyl and carbonyl ligands to give the acyl intermediate occurs 38 times slower for [RhMeI 2 (CO)(PEt 3 ) 2 ] than for [RhMeI 3 (CO) 2 ] Ϫ but the steady state concentration of the intermediates is different in that [Rh(COMe)I 2 (PEt 3 ) 2 ] is thermodynamically less stable than [RhMeI 2 (CO)(PEt 3 ) 2 ]. In CH 2 Cl 2 , [Rh(COMe)I 2 (CO)(PEt 3 ) 2 ] reductively eliminates MeCOI. [RhI(CO)(PEt 3 ) 2 ] reacts with CO to give [RhI(CO) 2 (PEt 3 ) 2 ]. Catalyst degradation occurs via [RhHI 2 (CO)(PEt 3 ) 2 ], formed by oxidative addition of HI to [RhI(CO)(PEt 3 ) 2 ], which reacts further with HI to give [RhI 3 (CO)(PEt 3 ) 2 ] from which [Et 3 PI] ϩ reductively eliminates and is hydrolysed to give Et 3 PO. In the presence of water, much less [RhI 3 (CO)(PEt 3 ) 2 ] and Et 3 PO are formed so the catalyst is more stable, but loss of [Et 3 PMe]ϩ and [Et 3 PH] ϩ from [RhMeI 2 (CO)(PEt 3 ) 2 ] or [RhHI 2 (CO)(PEt 3 ) 2 ], respectively, lead to catalyst deactivation. The rate determining step of the catalytic reaction in the presence of water is MeI oxidative addition to [RhI(CO)(PEt 3 ) 2 ], but in the absence of water there is evidence that it may be reductive elimination of MeCOI from [Rh(COMe)I 2 -(CO)(PEt 3 ) 2 ]. [RhMeI 2 (CO)(PEt 3 ) 2 ] has mutually trans phosphines and the methyl group trans to I.

Under mild conditions, [RhI(CO)(PEt 3 ) 2 ], is more active for the carbonylation of methanol to ethanoic acid than [Rh(CO) 2 I 2 ] 2 , which is widely used industrially; intermediates in the catalytic cycle have been identified and characterised.

Rare q2-benzyne and benzylidene complexes of niobium have been synthesised from [Nb(C5R5)(NR')C12] (R = H, Me; R' = 2,6-Pri2C6H3) via intermediate diphenyl and dibenzyl species and their molecular structures have been determined: these complexes are analogues of well-established zirconocene derivatives.Recently? we described a series of half-sandwich imido useful relationship since zirconocene reagents are widely complexes of niobium of the general type [Nb(C5R5)-exploited in organic synthesis2 and polymerisation ~atalysis.~ (NR')]C12] (R = H, Me; R' = 2,6-Pri2C6H3, But, Me),l and Herein, we report the synthesis and characterisation of rare noted the similarities between the shapes and energies of the examples of benzyne and benzylidene complexes of niobium frontier orbitals of the [Nb(CSRs)(NR')] fragment with those which are direct analogues of synthetically useful zirconocene of the bent metallocene unit [Zr(C5R5)2]. This is a potentially derivatives.

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