Group 13 Chemistry - American Chemical Society

Chapter 4

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Metallaboranes of the Earlier Transition Metals: Relevance to the Cluster Electron Counting Rules Thomas P. Fehlner Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556

Structures of the known hypoelectronic metallaboranes of tungsten and rhenium are used to explore the applicability of the electron counting rules to metallaborane clusters containing earlier transition metal fragments. It is demonstrated that for metallaboranes with formal skeletal electron pairs (sep) less than the p + 1 closo count, p = the number of vertices, the cluster shapes adopted are intermediate between the most spherical deltahedral shapes found in the closed borane anions (canonical structures) and the multi -capped skeletons found in metal systems. For a given cluster size the total connectivity, or number of bonded edges, is a constant and independent of the skeletal electron count. What varies is the distribution of vertex connectivities with the hypoelectronic skeletons exhibiting a greater number of high (occupied by the metal atoms) and low (occupied by the boron atoms) connectivity vertices. Consequently, as the number of sep falls below the p + 1 count, the average coordination number of the metals increases and the average coordination number of the boron atoms decreases. For each missing sep, the observed shape is related to the canonical form by one, in some cases more than one, diamond-square-diamond

© 2002 American Chemical Society

In Group 13 Chemistry; Shapiro, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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rearrangement or a cross cage metal-metal bond. In the specific case of the rhena- and tungstaboranes, it is shown that two of the missing sep are delocalized over the cluster bonding network and one is localized on the metal centers in the form of a Re-Re bond.

Metallaborane chemistry has been reviewed a number of times and the area is well defined in terms of structural chemistry//-7) With a few notable exceptions, these compounds can be described using the skeletal electron counting rules developed for the polyhedral boranes along with the isolobal relationships between transition metal fragments and main group fragments.fS13) Recently, a more general electron counting rule that interrelates boranes, heteroboranes, organometallic complexes and various condensed structures has been described.(14) However, despite the large number of structurally characterized compounds and the predictive capabilities of the counting rules combined with the isolobal analogy, significant challenges still exist. First, the earlier synthetic routes utilized lacked generality and, in many cases, selectivity. A consequence of the former is that the majority of metallaboranes characterized in the earlier work contain almost exclusively late transition metal fragments (group 8-10). A consequence of the latter is that the known reaction chemistry of metallaboranes is extremely limited. As we have described elsewhere, the large number of known monocyclopentadienyl metal halides constitute a potential source of metallaboranes of all the transition metals.(75,/6) In fact, the utilization of L1BH4 and B H 3 T H F as both activating agents and sources of polyborane fragments permits the formation of metallaboranes containing metals from groups 5 - 9 from the first to the third row of the transition metals. The reactions are generally selective giving good yields without complex separation steps. Thus, both structures of metallaboranes of the earlier transition metals and reaction chemistry can be investigated in a systematic fashion for the first time. The series of compounds C p * M B H , M = Cr,(77) Re,(7S) Ru,(79) Ir,(20) in which the metal ancillary ligand and the number of "extra" hydrogen atoms are constant is particularly interesting. Equally interesting, although incomplete, is the monometallic series C p * M X B H g , M X = T a C l / 2 7 ) WH ,(22,25) Co.(24,25) In a third series, C p * 2 M B H 9 , M = Cr,(2

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1

(C Me )RuHB H

[(C Me )RiiHB H ] Downloaded by NORTH CAROLINA STATE UNIV on September 18, 2012 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0822.ch004

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dsd

(C H )RuCp* Ru B H

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MLJ

Figure 8. The connections between the observed deltahedra for [(ifC Me )RuB H ] \ (tf-C Me )RuB H and {Cp*RuJ {V -C H )Ru}B H } in which thefirstcompound displays the canonical deltahedron for ρ = 10. 2

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tvc = 60

sep -12

sep = 9

Figure 9. The generation of the observed deltahedra for Cp*2Re2& l(fi10 7

an

d

Inji " from the canonical deltahedra for ρ = 12 and 11. The bold lines outline the diamond that undergoes rearrangement in thefirstcase.


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rearrangements generate two vertices of connectivity seven thereby flattening out the cluster sufficiently such that a Re-Re bond is possible. Likewise, in the hypoelectronic naked clusters of group 13 elements, the connection between the number of dsd rearrangements required and the electron count is not straightforward, e.g., I n n " with ρ - 2 sep requires five dsd rearrangements taking four vertices up one in connectivity and four down one to generate the observed structure from the canonical 11 vertex deltahedron.(44) Note that this is not the rearrangement described in the original work albeit equivalent to it. Unfortunately, these exceptions limit the usefulness of the analysis described even though in all cases the qualitative change in distribution of vertex connectivities is the same. Implicit in the capping principle is the existence of capped nido and arachno structures. In fact these are often observed in metal systems.(7i,50) Thus, a capped square pyramidal cluster, sep =7, is an alternative structure to an octahedron. Further a bicapped butterfly (edge-fused tetrahedra), sep =7, is an additional alternative. Likewise, Kennedy has described both isonido and isoarachno metallaboranes with ρ + 1, and ρ + 2, sep respectively.(51,52) Therefore, in principle, open analogs of the rhena- and tungstaboranes should also exist with ρ - 1 (nido analog) and ρ (arachno analog). We have recently characterized Cp*2Re2H2B7H9 as an arachno analog of Cp*2Re2B7H7 with 9 sep by the expedient of demonstrating that its tvc = 36 is the same as the preferred structure for an arachno 9 vertex canonical structure.(5JJ It is important to note that the additional four hydrogen atoms on the Cp*2Re2B7U7 framework do not simply convert it into an isocloso (hypercloso) structure but rather into an open structure that retains features favored by the metal fragment. Although the requirement of a Re-Re bond is important, the demands of the bridging hydrogen atoms must play a role—one analogous to the role they play in the octahedral structure of [Os6(CO)i8] " vs the capped square pyramidal structure of H Os (CO)\$.(54)


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Molecular Orbital Model of Hypoelectronic Metallaboranes of Tungsten and Rhenium Finally, the question why two cluster geometries with equal tvc's can have differing numbers of cluster bonding orbitals must be addressed. The origin of the capping principle has already been discussed in detail by Mingos as have the variation in number of cluster bonding orbitals in polar deltahedra, high nuclearity clusters with more than one shell, and clusters with metals associated with π-donor ancillary ligands.(72,55,56) A l l these ideas are clearly related to the present problem but either not sufficiently flexible or detailed enough to accommodate the series of compounds under discussion.


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In their exploration of physical factorization of the secular equation for various deltahedra, Hoffmann and Lipscomb considered a ring-polar separation in which the equatorial ring is capped by two polar fragments to form the complete cage.(57) Used effectively recently by others,(5S,59) this fragmentation connects cage structures to those of so-called triple-decker complexes where the polar fragments are now metal fragments such as C p M . We have already presented an analysis of the 24 electron { Ο ρ * & } 2 { μ - η : η C3H6C2B4H4} triple-decker (60) in which we showed that formation of the CrCr bond destabilizes and empties two in-plane ring orbitals that are metal-ring antibonding while concurrently stabilizing a cross cage M M bonding orbital and destabilizing (and emptying) its antibonding partner. This constitutes one example of a triple-decker sandwich complex, many of which have been characterized for a range of transition metals.f61) The variation in the structures of these compounds with d electron count is well understood.(62j The parent compound of the Cp*2Re2B H series is the unknown C p * 2 R e 2 B 6 H 6 , which we have recently characterized as the dichloroderivative.(63j However, {Cp*Re} ^^ :il -B4H4Co2(CO)5} is known and constitutes an isolobal analog of Cp*2Re2B6H6 (Figure 10). (64) Comparing 9 sep [{Cp*Rh}2{μ-η :η -C4H4B2Me2}] f65 ) with 6 sep C p * R e B H shows that as the C p * M fragments are moved within bonding distance three orbitals, which are filled in the 9 sep compound, are emptied and a substantial HOMOL U M O gap is produced for 6 sep.(57,62) These MO's consist of a M - M antibonding orbital and two M - B orbitals that are boron ring-metal δ


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(Cp*Re) B H 2

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(Cp*Re) B H4C02(C0)5

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s

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(Cp*Cr)2B H4C2(C3H )

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Figure 10. Comparison of Cp*2^2^6^6 Cp* Re2B4H Co2(CO)5 and

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a

triple-decker complex with known Cp* Cr2B4H C2C H . 2

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antibonding (Figure 11). Now the view of Cp*2Re2B H as pseudo tripledecker complexes (Figure 2) begins to make even more sense. That is, the generation of the observed structure of (Cp*Re)2B7H7 from the canonical tricapped trigonal prismatic geometry causes three orbitals to rise sufficiently high in energy to become unoccupied.(3/,6

Group 13 Chemistry - American Chemical Society

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