Alkyne-substituted clusters

Nuclear magnetic resonance spectroscopy, 46 161-162, see also specific compounds of alkyne-substituted clusters... 

Of special relevance to the investigation of alkyne-substituted clusters is the observation of a low-field resonance, generally in the range —1 to + 1.5t, for a proton attached to a carbon atom which is either a or n bonded to the metals. Values of the chemical shift for this signal for 23 tri- and tetranuclear osmium clusters were presented by... 

C NMR is generally applicable to organometallic and organo cluster compounds, but has the advantage of giving a direct probe on the acetylenic carbon atoms in alkyne-substituted clusters. With the development of more powerful NMR instruments, this technique has been used extensively to characterize organo-substituted cluster complexes in solution. For smaller clusters, the combination of 13C NMR with 31P NMR and resonance studies from metallic nuclei, where appropriate, frequently leads to complete structure elucidation. 

Regrettably, alkyne-substituted cluster complexes seem particularly prone to fragmentation and very few accurate mass spectroscopic studies have been reported. A recent exception has been the field-desorption and electron-impact mass spectral investigation of mono-and oligo-nuclear ferracyclic ring systems of the from Fei(CO)J,(C2R2)2 (x = 1, 2, 3 y = 6, 8) (384). These species show intense molecular ion peaks, which enable ready recognition of the molecular composition. 

The main reason is that all the systems are relatively complex and, while spectroscopic techniques may give part of the answer as to the nature and stereochemistry of the compound, a full crystallographic study will in a vast majority of cases give a definitive answer. In the reviews on alkyne-substituted clusters and related compounds, a large proportion of the discussion of the chemistry has been based on solid-state structural data, and this review is no exception. The variety of structural cluster types incorporating alkyne ligands will be presented in Section IV. 

In order to ease the interpretation of the photoelectron spectra, and to obtain the best correlation between experimental data and related theoretical calculations, it is advisable to consider a series of related molecules. In the case of alkyne-substituted clusters He(I)-excited vapor-phase photoelectron spectra have been obtained on the series of molecules Co2(CO)6(PhCCH) (1) (389), Fe3(CO)9(EtCCEt) (2) (390), M3(CO)9(/i-H)(CCR) (M = Ru, Os) (3) (391), and Co4(CO)10(PhCCH) (4) (389), which exhibit the range of bonding modes illustrated in Fig. 11. 

For alkynes bonded to higher nuclearity clusters no overall molecular orbital treatment encompassing all the variations in geometry has appeared yet. However, there are a small number of examples of specific alkyne-substituted clusters which have been analyzed by one type of molecular orbital treatment or another, and a number of these have been mentioned in Section III,G because photoelectron spectroscopy has been used as an aid to assignments. CNDO calculations (397) on Fe3(CO)9(EtCCEt) (390) and M3(CO)9(/i-H)(CCR) (M = Ru, Os) (391) and Fenske-Hall calculations (398) on Co4(CO)10(PhCCH) (389) indicate that there is net back donation into alkyne n orbitals, which increases as the number of metal atoms to which the ligand is bonded increases. The normally accepted view of considering the interaction... 

More generally, in terms of electron counting, alkyne-substituted clusters may be treated in a manner similar to other cluster systems. In these schemes the ligands are considered primarily as species which donate electrons to the cluster framework and do not have specific donor and acceptor properties of their own. Because of possible... 

Alkyne-Substituted Clusters Which Adopt Bonding Mode M12... 

There are two examples of alkyne-substituted clusters in which there is an interaction between the alkyne and five-metal centers. Both occurrences are found in the related pentanuclear ruthenium complexes Ru5(CO)14(/i-PPh2)(C=CPh) (431) and Ru5(CO)12(p-... 

Thermal activation of alkyne-substituted clusters frequently results in the loss of one or more carbon monoxide ligands (418, 445, 446). Concomitant with this loss is an alteration in the bonding mode of the organic ligand in order to retain the electron balance within the molecule (107). Such a reaction is shown in Fig. 41, where an osmacyclopentadiene ring is transformed into a trisubstituted-f/5-cyclopentadienyl system. Metal-metal bond formation may take place in some examples (446, 447). 

Variable-temperature H NMR studies of the reactions between alkyne-substituted clusters and protic acids indicate that protonation takes place initially at a metal center in the cluster framework with inter- and intramolecular hydride exchange. If a second protonation takes place, as happens in some cases, the site of the reaction is the organic ligand (118, 456, 457). 

The reactions of alkyne-substituted clusters with trialkylphosphines or phosphites that have been reported can be divided into two groups. In... 

Reactions between alkyne-substituted clusters and other metallic species have been used frequently as synthetic routes to mixed-metal clusters, particularly for Ru-Ni (214-217), as exemplified by Eq. (22), and Ru-Fe (220) complexes. In all these reactions the new metallic group forms bonds with the organic unit and with the metallic framework. It is possible that the first step in these reactions is the... 

Fig. 43. Reactions of alkyne-substituted clusters with mononuclear complexes and other groups.

Recently, a series of triruthenium clusters containing allenyl or alkynyl ligands have been investigated electrochemically, and two, subsequent one-electron reduction steps have been observed (471). Further studies of this kind should provide a new insight into the electronic processes which take place in alkyne-substituted cluster systems. 

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