Metal carbonyls structural chemistry

The foregoing examples show the relevance to metal-carbonyl cluster chemistry of the borane-oarborane structural and bonding pattern. Its relevance to other areas of chemistry may be explored readily using a systematic skeletal electron-counting procedure (161, 201). 

The chemistry of transition metal carbonyls structural considerations... 

These early measurements stimulated my interest in NMR spectroscopy, and, on moving to the University of Kent at Canterbury (1972), we were lucky to be able to buy the first Fourier Transform NMR spectrometer in the UK. This instrument was still based on an electromagnet ( H, 100 MHz) but allowed faster acquisition of NMR spectra and enabled the development of multinuclear NMR spectroscopy. This permitted me to start collaborating with Paolo Chini who had taken up an appointment at the University of Milan where he was developing metal carbonyl cluster chemistry. In Milan, Chini had access only to an IR spectrometer that aided the clean preparation and subsequent crystallisation of clusters, and, importantly, an X-ray diffractometer for their structural characterisation. 

The characterization of structure and dynamics in metal carbonyl cluster chemistry has understandably been based around single-crystal X-ray diffraction and NMR spectroscopic studies, respectively. As has been shown in the earlier part of this chapter, these sources of data can be used in conjunction to provide a coherent picture of the possible molecular configurations of a given species and the processes by which they interconvert, at least in favorable cases. In general however, it is not possible to determine the geometric structures of these species in solution, nor to obtain direct experimental evidence to confirm that the same structure(s) obtain in solution as in the solid state. Fortunately the rebirth and exploitation of X-ray absorption spectroscopy (XAS) and in particular the application of synchrotron X-ray sources and improved methods of data analysis to EXAFS (Extended X-ray Absorption Eine Structure) spectroscopy has provided exactly this sort of experimental evidence. In this section we provide selective coverage of such work as relates to the structure and dynamics of metal carbonyl clusters. 

The adherence to close-packed structural arrangements lends support to the idea that these compounds can be used as models for metal surface chemistry—with respect to chemisorbed species and their mobility and reactions of substrates on these surfaces. It also indicates a marked deviation from the behavior of boranes and their derivatives. Structures based upon some polyhedra favored by boron, such as the pentagonal bipyramid, triangulated dodecahedron, and especially the icosahedron, are absent so far in metal-carbonyl cluster chemistry. In this connection, it has been mentioned that [M(CO)3],g compounds should be the closest analogs to On skeletal electron counting... 

Zr(CH2Ph)4(CO)2] does not exist as almost all the mono- and bimetallic metal-carbonyl structures in which the metals do not have non-bonding elections (NNBE = 0) (the M-CO bond needs it backbonding from some filled metal d orbitals). There are only exceptions in lanthanide chemistry in which the metal-CO bond is very weak (R. Anderson s work). 

Since new carbonyl compounds of Ti, Zr, and Hf and their accompanying chemistry are appearing at a steady rate in the literature, it seems appropriate at this time to bring this rapidly growing area into perspective. In this review we will endeavor to categorize all reported group 4B metal carbonyls including their synthesis, formation,1 structure, spectroscopic... 

Hydrogen forms an extensive series of compounds with the metal carbonyls, and the nature of the H bonding within these complexes has been a point of debate for a considerable period. Both the chemistry and structure of metal carbonyl hydride compounds have been exten-... 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

The electronic structures of the metal carbonyls have been investigated vigorously over the past few years because the interaction between CO and metals (76, 137) is essential for an understanding of both organometallic chemistry and CO chemisorption phenomena. There is continuing debate concerning the relative importance of o-donation from the carbon lone pair of carbon monoxide and back donation from metal d electrons into vacant rr ligand orbitals. The most frequently cited data in favor of n back donation are... 

The early workers in coordination chemistry were more interested in the theory of bonding and structure than in any practical usefulness which the compounds might have. In more recent times, however, applications have developed. Perhaps the most important of these is in catalysis, especially for hydrogenation and the activation of carbon-hydrogen bonds. Metal carbonyls and their derivatives have played a large part in this application, as well as in carbonylation reactions such as the recently developed process for converting methanol to acetic acid 42... 

One aspect of metal carbonyl chemistry that should be mentioned in surveying the more commonly found modes of CO coordination is the stereochemical nonrigidity of carbonyl clusters. This aspect has received considerable attention over the past decade, especially as 13C nmr instrumentation has become more readily available. In many carbonyl clusters, terminal and bridging carbonyls as established by x-ray structural studies are equilibrated on the nmr time scale (37, 39-41). The manner of equilibration takes place in a concerted way in order that each metal center maintains a constant electron count. For example, bridge terminal interconversion, (1), proceeds via complementary unsymmetrical CO bridges. 

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