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ligands trans to the carbyne and the alkene, respectively, and the pyridine ligand trans to carbon monoxide are ~b t a i n e d . '~.~~ The neutral complex C1(C0)2(py)2W=CPh (1) reacts with maleic anhydride and fumaronitrile in T H F to afford the neutral complexes Cl(py),(CO)(alkene)W=CPh, 9 and 10 (9, alkene = maleic anhydride; 10, alkene = fumaronitrile), in 70-90% yield.I4 Complex 1 is significantly less reactive than its anionic counterpart 3. A large excess (10 equiv) of the alkenes and slightly elevated temperatures (40-50 "C) are required for this reaction to proceed at convenient rates.The molecular structure of the maleic anhydride complex 9 is shown in Figure l.I5 It is that expected16 for a chloro tungsten carbyne complex containing two donor ligands (py) and two acceptor ligands (maleic anhydride and carbon monoxide) with the chloride trans to the carbyne and the pyridine ligands trans to maleic anhydride and CO. The alkene double bond is perpendicular relative to the tungsten-carbyne bond1' thus maximizing a-bonding to the metal of both the alkene and the carbyne ligand. The anhydride group of maleic anhydride is oriented toward the carbyne ligand.The most surprising feature in the reactions leading to the tungsten alkene carbyne complexes is the facile loss of carbon monoxide-normally, substitution of a carbonyl ligand in bissubstituted tungsten carbyne complexes X(CO)2(L)2W=CR is difficult.I8 Therefore, we postulate an indirect mechanism for the substitution of carbon monoxide (eq 3). In the first step oneof the donor ligands (L = CI-, py) is substituted by the alkene ligand affording a neutral intermediate with the alkene ligand trans to one of the two carbon monoxide ligands. Subsequently, the CO trans to the strongly a-bonding alkene is labilized and substituted by the ligand L (L = C1-, py) giving rise to the observed products. Faster reaction of the anionic complexes 3 and 4 over the neutral complex 1 indicates that the first step is rate-determining. Lack of reaction of unactivated alkenes may be associated with the second step. Labilization of CO apparently is only achieved by electron-poor olefins such as maleic anhydride or fumaronitrile.

The solvolysis rates and products of the 6‐exo‐substituted 2‐exo‐ 1a‐1u, and 2‐endo‐norbornyl p‐toluenesulfonates 2a‐2u, have been determined. In general, the rate constants for 1 and 2 (log k) correlate well with the inductive constants σ Iq of the substitutents at C(6); however, their sensitivity to σ Iq is much larger in the 2‐exo‐series 1 than in the 2‐endo‐series 2. This differential transmission of polar effects is the cause of decreasing 2‐exo/2‐endo rate ratios from 2388 for R = t‐C4H9 to 0.37 for R = Br, i. e. with increasing electron attraction by the substituent. The high sensitivity of the rate constants for the 2‐exo‐p‐toluenesulfonates 1 to σ Iq indicates an unusually strong inductive interaction between C(6) and the incipient cationic center at C(2). This interaction is ascribed to the participation of the pentacoordinate C(6)‐atom, i. e. to 1,3‐bridging, a consequence of steric hindrance of nucleophilic solvent participation in norbornanes. Donor substituents enhance 1,3‐bridging, lead to faster reactions and to the formation of 2‐exo substitution products. Conversely, acceptor substituents reduce 1,3‐bridging, decrease rates and facilitate the formation of 2‐endo substitution products. Graded 1,3‐bridging is discussed in the light of Winstein's nonclassical ion concept.

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