Metalloborane

Deshalb kann man in polyedrischen Boranen BH-Einheiten durch Fe(CO)3 ersetzen, ohne die Struk-tur zu verandern. 

C2B3H5 besitzt die gleiche Struktur wie [BsHs] , C2B3H5Fe(CO)3 die gleiche wie [B6H6] und C2B3H5(Fe(CO)3)2 die gleiche wie [BtHt] . 

Sieben Elektronenpaare (n+1) stehen fiir die Cluster-Bindung zur Verfugung, und daraus resultiert eine oktaedrische Struktur (sechseckiges c/o.y(9-Polyeder). 

Ebenso wie Fe(CO)3 sind auch [Co(CO)3] und [Ni(CO)3] (alles d M(CO)3-Einheiten) isolobal zu BH, und eine Vielfalt von Metalloboranen kann hergestellt werden. 

CoCp ist isolobal zu [Co(CO)3] (CoCp kann auch durch Co Cp dargestellt werden, wobei Cp ein 6e-Donor ist, der drei Koordinationsstellen besetzt, s. Kap. 7.4.12), und es konnen Cobaltobora-ne dargestellt werden, die diese Gruppe enthalten, z.B. CpCo(C2B9Hn). 

---

Boron is similar to carbon in that it has a capacity to form stable covalently bonded molecular networks. Carbonates, metalloboranes, phosphacarboranes, and other families comprise thousands of compounds. 

Replacement of a (BH) unit in B5H9 by an isoelectronic organometallic group such as (Fe(CO)3 or (Co( j -C5H5)) can also occur, and this illustrates the close interrelation between metalloboranes, metal-metal cluster... 

The mles can readily be extended to isoelectronic anions and carbaboranes (BH=B =C) and also to metalloboranes (p. 174), metallocarbaboranes (p. 194) and even to metal clusters themselves, though they become less reliable the further one moves away from boron in atomic size, ionization energy, electronegativity, etc. 

The polyhedral metalloboranes, Part I metalloborane clusters with seven vertices and fewer. J. D. Kennedy, Prog. Inorg. Chem., 1984, 32, 519 (312). 

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

Formation of group-IIIB-group-IB and -IIB element bonds is limited almost exclusively to the element B and is observed predominantly in metalloboranes and metallocarboranes. The boron-IB and boron-group-IIB bonding interactions either are of the a type or can be described as multicenter bonds ... 

Copper and gold carborane complexes are discussed with the metalloboranes ( 6.5.3.4.). 

The lower borates, e.g., a-2,3-[(Ph3P)ClCd]B5Hjj and jU-2,3-[(Ph3P)ClCd]-B5H7Br, also form metalloboranes with group-IIB elements ... 

Metallocarboranes and metalloboranes, discussed in 6.5.4, may be prepared from metal halides and carborane anions. Selected examples of complexes formed by thermal—or otherwise—induced polyhedral rearrangements of existing metallocarboranes are given in a few equations, and some other individual compounds are listed in tables. 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

Synthetic routes to metalloboranes are briefly summarized. Two specific aspects of the field are selected for more detailed comment (i) the occurrence of exopolyhedral cycloboronation of P-phenyl groups (ii) oxidative cluster closure reactions. These topics are illustrated by reference to reactions of closo-BioHio nido-B9H12 and arachno-BoHiU" with a variety of Ir1 complexes, and several novel metalloborane cluster geometries (as determined by X-ray diffraction analysis) are described, notably nido-TIr(BoHi )H (PPh3)2], i so-ni do-[ IrC(OH)B8H6(OMe) (CgH PPh )... 

Numerous routes have been devised for the synthesis of metalloboranes. Among the most useful of these are ... 

U. Expansion, degradation, or modification of a preformed metalloborane cluster. 

Professor Greenwood has provided routes to metalloboranes which are derived from the noble metals Pt and Ir and the nido-B9H12 ", arachno-B9H1A, and closo-BioH102 ions. We wish to make note of some new reactions of Ir and Rh complexes with the arachno-B< H9 ion and compare these results with his observations of arachno-B9H14. ... 

In constrast to the previous section, the literature in this area over the period of the review was dominated by metalloheteroborane (mainly metallocar-borane) chemistry, narrowly focussed on charge-compensated carbollide C2B9H10L derivatives, with relatively few examples of metalloboranes. In... 

Metalloboranes, 4 172 exopolyhedral, 4 208-210 main group element, 4 207-208 transition element, 4 205-207 Metallo-carbohedrene clusters, 4 648 Metallocarboranes, 4 170 as catalysts, 4 217-218 economic aspects, 4 229 exopolyhedral, 4 215-216 f-block element, 4 225-226 host-guest chemistry-carborane anticrowns, 4 216-217 structural systematics, 4 176-179 transition metal, 4 210-215 Metallocene catalysis, MAO in, 16 92-93. 

These rules can be extended to isoelectronic anions, carbaboranes, metalloboranes, etc. (see also 4.4.4 fr). 

Some interesting photochemical reactions involving metalloboranes and metallo-carboranes have been reported. These include synthesis, isomerization and complexa-tion reactions. Gaines and Hildebrandt have used the following process to synthesize a metalloborane ... 

They have also demonstrated a reversible photochemical conversion from a tridentate to a bidentate B3H8 (Fig. 14). Fehlner has achieved a photochemical carborane synthesis by irradiating mixtures of B4H8Fe(CO)3 and dimethylacetylene. Products of the reactions include (CH3)4C4B4H4 and (CH3)8C8B4H4. Leyden and co-workers have observed two examples of metalloborane isomerization reactions initiated by light ... 

Franz and coworkers reported the photochemical synthesis of a metalloborane from a carborane and Fe(CO)s (reaction 54). 

Metal atoms have fewer valence electrons than orbitals available for bonding and in this they resemble boron. The consequences of this idea are examined and it is shown that many metals with electronegativities in the range 1.6-2.4 (B = 2.0) can subrogate boron atoms as vertices in polyhedral clusters. Such metalloboranes are often much more stable than the parent boranes or borane anions. Not only can metals mimic boron in known cluster geometries but the flexibility thus introduced can lead to novel and previously unsuspected cluster geometries. The construction of macropolyhedral clusters containing 17-20 vertices is also described. 

---