Isolobal metal fragment

The npheld end of the range is marked by white phosphorns P itself that can coordinate to a transition metal either by a vertex atom or with both phosphorns atoms of a common edge bonding to one or two transition metal atoms. It is important to realise that one or more phosphorns atoms in the P tetrahedron can be replaced by an isolobal metal fragment (see Fig. 7.40), with only a moderate downheld shift (for a summary of the isolobal theory see the box story following Chap. 7). 

Although polynuclear alkyne completes are often prepared by reaction of the alkyne with a suitable metal cluster fragment, heteropolynuclear complexes 69 (Scheme 4-38) have been obtained also by isolobal metal fragment substitution, as noted previously [26]. Higher-nuclearity alkyne complexes also can be produced by the addition of various metal carbonyl fragments to a lower-nuclearity alkyne complex [119], A novel entry to heterobi- (and tri-)metallic neutral p-propargyl complexes (e.g., Fe/Mo) via protonation of trinuclear p-Ti, T1 -o-propargyl derivatives 70 was recently described by Wojcicki and coworkers [120,121]. 

The addition of transition metal fragments ML (L = two-electron donor ligand) across formally unsaturated metal-metal or metal-carbon bonds is a well-developed synthetic route to heteronuclear clusters (1,2,11,12,27) and has received theoretical justification from Hoffmann s isolobal principle (46). The addition of a PtL2 fragment across an M=M double bond may be considered as analogous to the reaction of a carbene with an olefin, resulting in a cyclopropane. The use of isolobal analogies in the directed synthesis of heteronuclear clusters has been reviewed (11,12,27). 

Since many metal fragments are isolobal with CH2, it should be possible to make a range of metallacycloalkanes. Metallacyclobutanes are well known as a class of compounds and serve as key intermediates in catalytic alkene metathesis.1 This reaction has gained great importance in recent years through the work of Grubbs2 and Schrock.3 Alkene metathesis has many applications in organic chemistry,... 

Isolobal relationships between main-group and transition-metal fragments. Connections to organometallic chemistry... 

Isolobal main-group and transition-metal fragments... 

In the first three chapters, instances were noted where the number, symmetry characteristics and occupation numbers of the frontier orbitals of a transition-metal fragment were similar to those of a main-group fragment. Such fragments are said to be isolobal to emphasize similar bonding capabilities. Since its enunciation by Hoffmann and Mingos, the concept has been used effectively for the analysis of both organometallic and cluster problems. Let s explore the idea in a more systematic... 

Exercise 4.1. Fragment ethane into two CH3 radicals. Construct a neutral M(CO)m fragment isolobal with CH3. Give the structures and compositions of all compounds analogous to ethane that can be formed with these two isolobal main-group and transition-metal fragments. 

Exercise 4.4. Suggest a CpM fragment isolobal with a BH fragment. Systematically replace one and two BH fragments in B5H9 with this metal fragment and enumerate all isomers. 

The principal bonding interaction occurs between metallic u and cr frontier orbitals and the a C2 orbitals. You expect this because the [Mn(CO)s]+ fragment is isolobal with [CH3]+ and the product is isolobal with 2-butyne. However, the metal fragment provides the opportunity for weaker interactions between the tt-symmetry orbitals. These are of two types. A backbonding interaction (occupied metallic 77 orbitals with unoccupied C2 tt ) transfers charge from the metal to the C2 unit, thereby reducing the C-C bond order. For this compound, the observed C-C distance is consistent with a bond order of three hence, the interac-... 

Additional tetrahedrane clusters are formed when vertices in Co4(CO)i2 are substituted with isolobal (see Isolobal Analogy) (or nonisolobal) metal fragments. In this category are the mixed Co-Rh and Co-Ir carbonyl complexes, which can be formed from the reaction of M4(CO)i2 (M = Rh, Ir) and Co2(CO)g. For example, Co3Rh(CO)i2, which has been used as a hydroformylation catalyst precursor, is prepared in this way. Furthermore, since the Co(CO)3 group is isolobal... 

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