Early-transition elements

Cu ( j -C5H5)2] is not. Likewise, Fe and Ni carborane derivatives are extremely stable. Conversely, metallocarboranes tend to stabilize lower oxidation states of early transition elements and complexes are well established for Ti", Zr , Hf , V , Cr and Mn" these do not react with H2, N2, CO or PPh3 as do cyclopentadienyl derivatives of these elements. 

The lanthanoids also form metal-rich carbides of stoichiometry M3C in which individual C atoms occupy at random one-third of the octahedral Cl sites in a NaCl-like structure. Several of the actinoids (e.g. Th, U, Pu) form monocarbides, MC, in which all the octahedral Cl sites in the NaCl structure are occupied and this stoichiometry is also observed for several other carbides of the early transition elements, e.g. M = Ti, Zr, Hf V, Nb, Ta Mo, W. These... 

Volume 3 Main Group Early Transition Elements... 

Hughbanks T (1995) Exploring the metal-rich chemistry of the early transition elements. J Alloy Compd 229 40-53... 

HVC s are formed mainly by the early transition elements, especially those in groups 6 and 7. Their stabilities are markedly "row-sensitive," with the light metals showing in general very little tendency to form clusters or multiple M-M bonds in higher oxidation states. 

General information on the coordination chemistry of the transition elements is contained in G. Wilkinson, R. D. Gillard and J. A. McCleverty, eds. Comprehensive Coordination Chemistry, Vols. 3 (Early Transition Elements), 4 (Middle Transition Elements and 5 (Later Transition Elements). These are detailed and authoritative accounts. In addition, periodical issues of Coordination Chemistry Reviews contain reviews of the current chemistry of the transition (and main group) elements. 

Reviews and texts relevant to the early transition elements which have not been referred to in the text include ... 

It is clear that while the early transition elements, Sc and Ti, have 3d orbital energies which are higher than the 2p orbital energy of C, the 3d orbital energies fall sharply across the row, with the later energies comparable to the 2p orbital energies of and F. 

D. L. Kepert, The Early Transition Elements, Academic, London, 1972. 

A thermally robust dihapto C02 complex was easily obtained according to equation (62).631 Complex (46) is the first authentic (X-ray diffraction) C02 complex of a hard oxophilic early transition element. The C02 (C—O bond lengths 1.283(8) and 1.216(8) A) lies close to the alkyl group, but its orientation is such that its insertion into the metal-alkyl bond would produce the unfavored metallacarboxylate ester instead of a carboxylato complex. 

The chemistry of carbene complexes is a field which has developed in the past 20 years. The current interest is primarily centered on the chemistry of the early transition elements, but much of the early work was carried out at low valent, metal centers. The major complexes formed by platinum are those with the metal in a divalent oxidation state. 

In another recent study of CO reduction using organometallic compounds of the early transition elements, Caulton et al. report that CH4 is evolved when toluene solutions of (C5H5)2Ti(CO)2 are heated at 150°C under H2 or H2 -l- CO atmospheres (55). When deuterium is used, the observed product is CD4, although some deuteration of the cyclopentadienyl rings is noted. Caulton does not report the percent yield of methane, but does state that the reaction is not catalytic. One of the most novel aspects of this investigation is the isolation of a blue Tig cluster that is inert to further reaction... 

The conversion of CO + H2 (syn-gas) to hydrocarbons and oxygenates (Fischer-Tropsch chemistry)119 is of considerable industrial importance and recently the activation and fixation of carbon monoxide in homogeneous systems has been an active area for research.120,121 The early transition elements and the early actinide elements, in particular zirconium124 and thorium,125 126 supported by two pentamethylcyclopentadienyl ligands have provided a rich chemistry in the non-catalytic activation of CO. Reactions of alkyl and hydride ligands attached to the Cp2M centers with CO lead to formation of reactive tf2-acyl or -formyl compounds.125,126 These may be viewed in terms of the resonance forms (1) and (2) shown below. 

C NMR Chemical Shifts for Selected Anionic and Neutral Metal Carbonyls of Some Early Transition Elements... 

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