Metal compounds, coordinatively unsaturated

The involvement of intermediates and transition states containing higher coordinate carbon has been originally proposed by Olah in both electrophilic reactions involving electron-deficient systems such as carbocations, heterocations, carbenes, nitrenes, silylenes, coordinatively unsaturated metal compounds, and so on, as well as in nucleophilic Sn2 reactions by Ingold. Whereas in electrophilic reactions, the pentacoordinate carbocationic centers are associated with eight electrons and thus can be intermediates (albeit... 

In this chapter, we successively review reactions of electrophiles, coordinatively unsaturated metal compounds, carbenes, nitrenes, and heavy analogs of carbenes (silylenes, germylenes, stannylenes) with C-H and C-C bonds. Discussion of some electrophilic reactions of ti-donor systems is also included, along with Sn2 reactions. Tlie emphasis in all these discussions is centered on the involvement of hypercarbon intermediates (or transition states) of the reactions. 

The spectroscopy, reaction kinetics, and photophysics of coordinatively unsaturated metal carbonyls generated in the gas phase via UV photolysis are probed via transient infrared spectroscopy. The parent compounds that have been used to generate coordinatively unsaturated species are Fe(CO)5, Cr(C0)5 and Mn2(CO)io- In contrast to what is observed in solution phase, photolysis of these compounds produces a variety of coordinatively unsaturated photoproducts. 

Hydrido complexes can be prepared by the oxidative addition of hydrogen to coordinatively unsaturated metals (e.g. Vaska s compound) or by reduction of higher valent compounds with borohydride or similar reagents. 

This type of template transformation includes mainly reactions of immediate interaction of reagents (Sec. 3.1) - coordinatively unsaturated metal-containing ligand systems (complex compounds) with Lewis acids and leads (3.198) to obtaining di-and poly-, homo- and heteronuclear structures. 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

A common method for the preparation of carbene complexes without heteroatom substituents is the reaction of a coordinatively unsaturated metal complex with a diazoalkane. This method is effective for the preparation of CpOsCl(P Pr3)(=CHPh) from CpOsCl(PTr3)2. The carbene in this complex is electrophilic and reacts with aUcyl and aryl lithium compounds and with Grignard reagents see Grignard Reagents) (Scheme 14). ... 

Optimal substrates for a synergistic [15,16] inner-sphere [5,12] interaction with the electronically ambivalent species L M-R (R=H, alkyl, aryl) have a similarly ambivalent acceptor/donor make-up They should have electron pair-donating heteroatoms such as O, N or S in order to form coordinative bonds to the coordinatively unsaturated metal center of the organometallic compound (Eq. (1)), at the same time, a low-lying unoccupied orbital such as the LUMO (w ) of a 7t system is required for the acceptor function. 

The kinetic parameters of the propagation of olefin polymerization on different active centers are compiled in Table 10. Apparently, for both the transition and non-transition metal compounds the insertion of the olefin into Mt—C bonds proceeds with the participation of coordinatively unsaturated metalalkyl compounds via intermediate n-complexes. The higher reactivity of transition metal compounds compared with organoaluminium compounds is primarily due to the lower activation energy of the propagation step when Mt is a transition metal. Many facts indicate that polarization of the Mt—C bond does not determine the reactivity of metalalkyl compounds in olefin addition, e.g. due to the decrease of reactivity in the order... 

One model for bonding in a diazo compound would be die ylide (89 equation 37)." Unlike many other ylides, diazoalkanes are stable to air and water. With acid, however, protonation can lead to the highly reactive salt (90), the functional equivalent of the corresponding carbocation. As the substituents on the diazo group are made increasingly electron withdrawing, the ylide becomes less basic, and thus more stable to acid. Reaction of a diazo compound with a transition metal can also often be understood as proceeding via initial donation of electron density by (89) to a coordinatively unsaturated metal center. 

Hydrocyanation of styrene 26 (eq. (7)) has been examined in some detail. With Ni[P(0-o-tolyl)3]4 27 the branched nitrile 29 is strongly favored over the linear one, which is explained by the intermediary formation of a detectable alkyl species 28. The stability of this intermediate is attributed to the donation of aromatic ring electrons to the coordinatively unsaturated metal center. Crystal structures of related compounds are reported in the literature [53, 54]. 

In general, an incompletely coordinated (coordinately unsaturated) metal atom in an M(CO) group will combine with a polyolefinic compound so as to complete its normal coordination requirement. It does so on the basis that each double bond is, formally, the equivalent of one 2-electron donor. Several categories may be distinguished among the many compounds of this type ... 

Thus, a system with a C-H — M agostic bond is, as it were, on the reaction pathway between the C-H -i- M system and the alkyl hydride derivative C-M-H. In recent years, compounds containing agostic bonds have been proposed as intermediates in reactions that involve C-H bond activation. For example, in the thermolytic rearrangement of cw-bis(silylmethyl)platinum(II) complexes, a mechanism is proposed which involves preliminary dissociation of one Pt-P bond compensated by an agostic interaction between tbe coordinatively unsaturated metal and a phosphine substituent (VI.2) [8]. 

There is some evidence of the involvement of the coordinatively unsaturated bimetallic compound [CoRh(CO)v] as an active species in hydroformylation reactions.l A synergistic effect was observed in the hydroformylation of pentafluorostyrene. i Excellent regioselectivities were reported for the hydroformylation step of the hydroformylation-amidocarbonylation of trifluoropropene using a [Co2(CO)8]/[Rh6(CO)i6] system in which [CoRh(CO)7] could have been generated. A mixed-metal [Co.vRh (CO).] species has previously been postulated as an active catalyst in the hydrocarbonylation of diketene. " Hydroformylation of 1-hexene and 3,3-dimethylbut-l-ene catalyzed by [Co2Rh2(CO)i2] and of... 

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