Main Group Element-Transition Metal Mixed Compounds

Unsaturated species such as the cluster [H4Re4(CO)i2] with 56 valence electrons can be rationalized by considering a parent hypothetical anion [Re4(CO)i2] with four face-delocalized, three-center/two-electron metal-metal bonds, each of which is able to coordinate one proton in the hydride. 

2 Main Group Element-Transition Metal Mixed Compounds 

In this section, a series of cluster species in which one or more vertexes are constituted by a main group element will be analyzed. Although many of these compounds have been discussed to some extent in the previous Chapter, it is 

The simplest and also the commonest among main group-transition metal mixed compounds are those in which the main group element caps a triangular array of metal atoms in any triangular or more complex deltahedral metal cluster thus forming units of the type (jU3-E)M3. 

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The 18-electron rale is not obeyed as consistently by these types of oiganome-tank compounds a by the carbonyl and nitrosyl complexes and their derivatives. For example, in addition to ferrocene. M(i 5-CsHs)2 compounds are known for most of the other elements of the first transition series (M — V, Cr, Mn.Co, Ni) and these cannot obey ihe 18-electron rule. However, only ferrocene shows exceptional thermal stability (stable to 500 C) and is not oxidized by air. Furthermore, cobaltocene, a 19-electron species, is readily oxidized to the 18-electron cobaltocenium ion. (Co(ip-CsH )3)4 , which reflects much of the thermal stability of ferrocene. Mixed cyclopentadienyl carbonyl complexes are common K -CjHjMCO) ]. [(if-CjH )-Cr(CO), . [( -CjHOMnCCOjJ, [(>r-C,H,>Fe(CO ,, . [fo -CjiyCoCoy. and (ip-CsH,)Ni(CO) 2. Of interest is the fact that among these compounds, the odd-atomic-number elements (V. Mn, and Co) form monomers and the even-atomic-number elements (Cr. Fe. and Ni) Ibrm dimers, which is in direct contrast to the behavior shown by the simple carbonyl complexes. Cyclopentadienyl derivatives are now known for every main group and transition metal of the periodic table and for most of the -block metals.89... 

Cluster Compounds Inorganometalhc Compounds Containing Transition Metal Main Group Elements Coordination Numbers Geometries Electronic Structure of Main-group Compounds Lead Organometallic Chemistry Mixed Valence Compounds Tin Inorganic Chemistry. 

Current concepts in modem chemistry/Transition metal cluster chemistry/Main group-transition metal mixed clusters/Cluster compounds of main group elements/ Synthetic analogues of the active sites of iron-sulfur proteins... 

Many formations and decompositions or other equilibrations of coordination compounds are extremely rapid. The half-life of a reaction such as the replacement ( substitution ) by ammonia of water coordinated to nickel(II) ions is typically microseconds to milliseconds, and there is indeed a convenient distinction (due to Taube) for reactions in solution between kinetically labile and kineti-cally inert systems. On mixing 0.1 M aqueous solutions of the reagents, labile equilibria are fully established within 1 min, whereas inert systems take longer. Many of the ions of the heavier (second and third row) transition elements in several oxidation states (e.g., both Pt + and Pt" +) are inert, as are many spin-paired d ions (Fe +, Co +, Ni" +) and chromium(III) in the first row. Kinetic lability in solution is the rule for coordination compounds containing main group metals. Reactions of solid coordination compounds (like most other solid-state changes) are usually slow. It is this kinetic inertness that has led to the isolation of so many metastable coordination compounds. 

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