Bonding cluster complexes

One of the m jor attractions in the metal-atom synthesis of dimer and cluster species is the ability to isolate highly unsaturated species, M Lm, that may then be considered to be models for chemisorption of the ligand, L, on either a bare, or a supported, metal surface (,100). It is quite informative to compare the spectral properties of these finite cluster-complexes to those of the corresponding, adsorbed surface-layers (100), in an effort to test localized-bonding aspects of chemisorption, and for deciphering UPS data and vibrational-energy-loss data for the chemisorbed state. At times, the similarities are quite striking. 

Novel boron-gold bonds containing complexes are realized in two borido clusters. Addition of [HFe4(CO)i2BH] ppn to xs (PPhjIAuCl in CH2CI2 forms the green-brown, air-sensitive Fe4(CO) 2(AuPPh,)2BH ... 

Mixed O, N donor molecules are truly extensive and structurally diverse, and only a few selected examples will be presented. In line with other 2-substituted pyridine analogs reported in this chapter, it is worthwhile noting the chemistry of 2-pyridone (or 2-hydroxypyridine, Hopy), which can form O-bonded monodentate complexes such as Co(Hopy)4(N03)2, but as the monoanion is an effective chelate ligand, forming Co(opy)2 and Co(bpy)(opy)2 compounds.454 An unusual solid state melt reaction with Co(OAc)2 yields the dodecanuclear cluster Co12 (OH)6(OAc)6(opy )12.455... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

Metal-metal (M-M) bonds, first noted in the early sixties, occur in several thousand transition-metal compounds [1]. Complex technetium compounds and compounds with M-M bonds (clusters) have been studied more extensively than many other classes of inorganic compounds. Increasing interest in technetium compounds is due to the practical uses of the "mTc isotope, which ranks first among radioactive isotopes used in nuclear medicine diagnostics [2-4]. On the other hand, technetium clusters are an interesting object for theoretical studies, because until recently, they were the only compounds in which the presence of these anomalous chemical bonds was thought possible. 

Supramolecular aggregations are commonly referred to by a variety of terms, including adduct, complex, and van der Waals molecule. In this chapter we shall primarily employ the more neutral term cluster, which may, if desired, be qualified with the type of intermolecular interaction leading to clustering (e.g., H-bonded cluster ). General and specific types of intermolecular forces are discussed in the following sections. 

There is a marked difference in chemical reactivity between bridging and terminal hydrogens. Terminally bonded hydrogens readily react in a similar manner to that observed for mononuclear hydrides. Thus reactions with chlorinated hydrocarbons such as carbon tetrachloride yield the chloro cluster complexes and chloroform. In contrast, bridging hydrides are stable and may be studied in chlorinated sol-... 

Very few examples of these species have appeared since the field was last reviewed.1 One species of interest is the seven-membered heterocycle 100, which was obtained from the dilithiated species 97 via the formation of a Ge=B doubly bonded intermediate (Scheme 30).206 A rare example of a germanium/gallium cluster complex has also been prepared (Equation (162)).207... 

As has been previously described, breaking of the central C—C single bond in 1,3-diynes has been reported in several systems, including complexes of groups 3 and 4 (Section VI) and cluster complexes of Group 8 elements (Section IV.D). 

The number of central atoms joined in a single coordination entity or cluster by bridging ligands or by metal-metal bonds. Such complexes are referred to as being dinuclear, trinuclear, tetranuclear, polynuclear, etc. 

Heating 26 with an excess of PPhj under pressure in chlorobenzene leads to the loss of one coordination site. One more Os-Os bond is broken upon formation of the complex Os3(CO)6(CNR)(p3-CNCH2QH4)-(PPh3)(p-PPh2)(p-T r -C6o) [74], The T r binding mode observed in this complex is very unusual for Cg0. One further example, where can act as a four-electron ligand is the fullerene carbido pentaosmium cluster complex 28 shown in Scheme 7.13 [75,76], The ri T -complex 28 is formed in a mixture with the normal T r r carbido pentaosmium complex 27. The two complexes can be converted into one another at elevated temperatures. 

In bridged metal-metal bonded dimeric complexes, the relative importance of metal-metal and bridging ligand effects are more difficult to unravel. Dahl and his co-workers have elegantly exploited systematic crystallographic analyses to detail the stereochemical consequences of valence-electron addition or removal in dimeric metal complexes (46, 47, 65, 230) and clusters (66, 88, 204, 205, 213, 216, 222). Their experimental work has been neatly underpinned by nonparameterized approximate Hartree-Fock molecular orbital calculations (217) on the phosphido-bridged dimers [Cr2(CO)80ti-PR2)2]n"2 and [Mn2(CO)g(/i.-PR2)2]n (rt = 0, + 1, or +2) ... 

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