Metal-borane clusters

Correlations between B NMR chemical shifts and electronic structure in borane and metal-borane clusters 99CCC767. 

Abstract Quantitative structure-activity relationship (QSAR) models are generated for biological activity and toxicity in terms of global and local reactivity descriptors within a conceptual density functional theory framework. Possible anticancer activity of two new metal-borane clusters is analyzed. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

Oxidative insertion of a metal into a borane cluster. 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

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... 

Mingos rules. This hypoelectronicity or electron poverty (fewer than the Wade-Mingos 2n + 2 skeletal electrons) in the bare metal cluster anions Enz (z < n + 2) leads to deltahedra not only different from those in the deltahedral boranes but also different from those in hypoelectronic metal carbonyl clusters of metals such as osmium. 

Another very useful rule for classifying the structures of polyboranes and hetero-boranes as well as many metal boron cluster compounds and their derivatives has been developed by Rudolph, Williams, Mingos and Wade (see Chapter 1.1.2) [4]. Today these are generally termed the Wade rules. They can be derived from the structures and electronic requirements of closed polyhedral boranes, such as an octahedron or an icosahedron, which are present in the anions B6H62 and B,2 H, 22. Since there are only exopolyhedral B-H bonds the number of electron... 

Presented in part at the Ralph W. Rudolph Memorial Symposium at the ACS Meeting, Kansas City, on September 15th, 1982. For a summary of some of the numerous valuable contributions of the late R.W. Rudolph to Inorganic Chemistry cf. "Borane Clusters, Metal Clusters and Catalysis" in Parry, R.W. Kodama,... 

That transition metal-carbonyl clusters, which contain an apparent abundance of electrons, might have much in common with boranes and carboranes, notorious for their deficiency of electrons, appears at first sight improbable. However, the structural and bonding relationship between them becomes apparent if one considers certain metal-carbonyl clusters for which localized bond treatments are unsatisfactory. 

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

Many further examples could be added to those already cited to support the close relationship between carbonyl clusters and borane clusters, particularly for species based on an octahedron. These other clo80 octahedral clusters include anions such as Co0(CO)i6 (34), [Fe6(CO)ieC] - (35, 40), [Rhe(CO)i5l]- (6), [Nie(CO)i2] - (31), [Co4Ni2(CO)i4] (5), and [Rhi2(CO)8o], in which two Rhg octahedra are linked by a metal-metal bond and two bridging carbonyl groups (2). 

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). 

Although most known metallocarboranes have only 1 metal atom per polyhedron, the existence of an increasing number of metallocarboranes with 2 or more metal atoms per cluster emphasizes the close relationship between metal clusters and borane clusters. Since they can be synthesized from cZoao-carboranes by replacing BH units by metal-carbonyl or metal-cyclopentadienyl residues, carborane clusters can effectively be used as disposable templates on which to fabricate... 

In addition to the metal-caxbonyl clusters discussed in Sections IV and V, which conform to the borane pattern, there is an increasing... 

Another class of molecules that will be discussed contains cluster compounds such as the polyhedral borane anions, B H -", and some metal containing species such as the metal carbonyl clusters. 

Much of the current literature on metal atom cluster species employs bonding concepts that are derived from MO treatment of the polyhedral borane anions, We thus begin by discussing these species, of which the most important examples are shown in Figure 8.15. We shall deal with the BaHg" ion in detail to illustrate the general approach to these systems. 

K. Wade, The Structural Significance of the Number of Skeletal Bonding Electron-pairs in Carboranes, the Higher Borane Anions, and Various Transition-metal Carbonyl Cluster Compounds, Chem. Comm. 1971, 792-793. 

---