Cyclopentadienylironphosphacarboranes: fragility of polyhedral edges in the 11-vertex system

Abstract

The lowest energy CpFeCHP(CH₃)B_n−3H_n−3 (n = 8 to 12) structures, including the experimentally known CpFeCHP(CH₃)B₈H₈, have been investigated by density functional theory. The central FeCPB_n−3 polyhedra in all of the lowest energy such structures are the most spherical closo deltahedra. The heteroatoms are so located to have adjacent iron and phosphorus atoms and non-adjacent phosphorus and carbon atoms. One of the Fe–B bonds from the degree 6 iron vertex in the 11-vertex CpFeCHP(CH₃)B₈H₈ structure appears to be fragile, readily elongating to ∼3.1 Å in one of the low-energy structures, consistent with experimental observation on this system.

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1. Introduction

The chemistry of metallaboranes was initiated by Hawthorne and co-workers in the 1960s,¹ first with 12-vertex icosahedral systems and later with 10- and 11-vertex polyhedral systems using decaborane, B₁₀H₁₄ as the boron hydride raw material. Shortly thereafter Grimes and co-workers² first synthesized smaller polyhedral metallaboranes with as few as six vertices using pentaborane, B₅H₉ as the boron hydride raw material. Most of the early work used polyhedral ligands derived from dicarbaboranes in a search for neutral species isoelectronic with the most spherical closo deltahedral borane anions B_nH_n²⁻ (n = 6 to 12).^3,4 Such species have 2n + 2 skeletal electrons according to the Wade–Mingos rules.^5–8

The early work on metallaboranes used a CpCo (Cp = η⁵-C₅H₅) vertex to replace a BH vertex in a polyhedral dicarbaborane structure. A CpCo vertex is isolobal with a BH vertex and, like a BH vertex, is a donor of two skeletal electrons. The early work on metallaboranes often involved stable neutral CpCoC₂B_nH_n+2 species. In subsequent work main group heteroatoms were introduced into the cages of such metallaborane structures. In this connection phosphorus can be considered as a “carbon copy”⁹ since a bare phosphorus atom donates three skeletal electrons like a CH vertex. Thus replacement of two CH groups by bare phosphorus atoms in the dicarbaboranes C₂B_n−2H_n gives the diphosphaboranes P₂B_n−2H_n−2. The icosahedral diphosphaborane P₂B₁₀H₁₀ was first synthesized in relatively low yield by Todd and co-workers in 1989.¹⁰ Much more recently the yield in the synthesis of P₂B₁₀H₁₀ has been greatly improved¹¹ so that it is now available in quantities as a reagent for the synthesis of metalladiphosphaboranes. In this connection an isomer of the cobalt complex CpCoP₂B₉H₉ has been synthesized from P₂B₁₀H₁₀ and structurally characterized by X-ray crystallography. Ferradiphosphacarboranes isomers of the stoichiometries CpFeCP₂B₈H₉ (ref. 12) and CpFeC₂PB₈H₁₀ (ref. 13 and 14) isoelectronic with CpCoP₂B₉H₉ have also been synthesized.

A complication in the development of the chemistry of metalladiphosphaboranes isoelectronic with metalladicarbaboranes is the basicity of the lone pairs on the phosphorus vertices. This is demonstrated by synthesis of the cobalt hydride HCo(η¹-P₂B₁₀H₁₀)₂(PEt₃)₂ derived from HCo(CO)₄ by replacement of two CO groups with Et₃P ligands and the remaining two CO groups by η¹-B₁₀H₁₀P₂ ligands bonding to the cobalt through a lone pair from one of the phosphorus vertices.¹⁵ The basicity of a vertex phosphorus atom in a polyhedral borane can be quenched by alkylation analogous to the conversion of a phosphine R₃P: to a phosphonium ion [R₃PR′]⁺. An RP vertex is a four-electron donor in a polyhedral borane structure. Thus species CpFeCHP(R)B_n−3H_n−3 have the 2n + 2 skeletal electrons for a most spherical deltahedral structure (Fig. 1) and are isoelectronic with B_nH_n²⁻, C₂B_n−2H_n, CpCoC₂B_n−3H_n−1, and CpCoP₂B_n−3H_n−3.

Fig. 1 The most spherical closo deltahedra having from 8 to 12 vertices. Degree 4, 5, and 6 vertices are colored in red, black, and green, respectively.

Sneddon and co-workers¹⁶ have used the anion [CH₃PCHB₈H₁₁]⁻ having a decaborane-like structure to synthesize the iron complex CpFeCHP(CH₃)B₈H₈ (Fig. 2). In the [CH₃PCB₈H₁₂]⁻ anion the carbon atom is located at a degree 4 vertex adjacent to the phosphorus atom located at a degree 3 vertex. However, in the reaction of the anion with a CpFe source the carbon atom migrates so that both the carbon and phosphorus atoms are located at the two degree 4 vertices of the resulting most spherical 11-vertex deltahedron, namely the so-called “edge-coalesced icosahedron” (Fig. 1 and 2). The iron atom in this structure is located at the unique degree 6 vertex of this deltahedron. However, one of the bonds from the iron atom to an adjacent boron atom is abnormally long (dashed line in Fig. 2) suggesting that a degree 6 iron atom cannot form strong bonds to all six adjacent atoms. This phenomenon suggested a more detailed study of this and related systems using density functional theory. In order to provide a broad perspective of the nature of these ferraphosphacarbaboranes, we included in this study the complete series of CpFeCHP(CH₃)B_n−3H_n−3 systems having 8 to 12 vertices.

Fig. 2 The phosphacarbaborane and its conversion to CpFeP(CH₃)CHB₈H₈. Unmarked atoms are boron atoms and external hydrogen atoms are omitted for clarity. The dashed line corresponds to an elongated Fe⋯B edge.

2. Theoretical methods

In order to reduce computational costs the full geometry optimizations were initially carried out on the CpFeCHPHB_n−3H_n−3 systems (n = 8 to 12) at the B3LYP/6-31G(d)^17–20 level of theory. The lowest energy structures were then reoptimized at a higher level, i.e., M06L/6-311G(d,p)²¹ and these are the results presented in the paper. The initial structures were chosen by systematic substitution of one BH vertex in B_nH_n²⁻ by a CpFe unit, then another BH vertex by a PH vertex, followed by all possible substitutions of the remaining BH vertices by a CH vertex. However, in order to keep the number of possible structures manageable, structures with degree 3 CpFe vertices capping a triangular face were excluded from consideration because of the general preference of transition metal units for higher degree vertices. The large number of different starting structures for the optimizations included 593 structures of the 8-vertex clusters CpFeCHPHB₅H₅, 439 structures of the 9-vertex clusters CpFeCHPHB₆H₆, 648 structures of the 10-vertex clusters CpFeCHPHB₇H₇, 938 structures of the 11-vertex clusters CpFeCHPHB₈H₈, and 304 structures of the 12-vertex clusters CpFeCHPHB₉H₉ (see the ESI†). It is thus assured that the number of polyhedral frameworks is representative enough and covers all geometries of interest. The natures of the stationary points after optimization were checked by calculations of the harmonic vibrational frequencies. If significant imaginary frequencies were found, the optimization was continued by following the normal modes corresponding to imaginary frequencies to insure that genuine minima were obtained. The PH vertices in the lowest energy CpFeCHPHB_n−3H_n−3 structures were then replaced by PCH₃ vertices and the structures reoptimized to give the CpFeCHP(CH₃)B_n−3H_n−3 structures reported in this paper and related to the experimental CpFeCHP(CH₃)B₈H₈ structure.¹⁴

All calculations were performed using the Gaussian 09 package²² with the default settings for the SCF cycles and geometry optimization, namely the fine grid (75 302) for numerically evaluating the integrals, 10⁻⁸ hartree for the self-consistent field convergence, maximum force of 0.000450 hartree bohr⁻¹, RMS force of 0.000300 hartree bohr⁻¹, maximum displacement of 0.001800 bohr, and RMS displacement of 0.001200 bohr. ¹¹B NMR chemical shifts were calculated at the gauge invariant atomic orbital (GIAO) – B3LYP/6-311+G(d,2p) level and referenced to BF₃·OEt₂.

The CpFeCHP(CH₃)B_n−3H_n−3 (n = 8 to 12) structures are numbered as B(n − 3)CPFe-x where n is the total number of polyhedral vertices, and x is the relative order of the structure on the energy scale (M06L/6-311G(d,p) including zero-point corrections). The lowest energy optimized structures discussed in this paper are depicted in Fig. 3 and 5–8. Only the lowest energy and thus potentially chemically significant structures are considered in detail in this paper. All of the structures reported in this paper are closed-shell structures with substantial HOMO–LUMO gaps ranging from 1.7 to 3.5 eV (Table S6 of the ESI†). More comprehensive lists of structures, including higher energy structures, are given in the ESI.†

Fig. 3 The three lowest energy CpFeCHP(CH₃)B₈H₈ structures. A dashed line indicates the long Fe⋯B distance in B8CPFe-2 and an arrow indicates the mobile boron atom interconverting B8CPFe2 and B8CPFe-3.

Fig. 4 Relative energy as a function of the fragile Fe–B bond length in the B8CPFe-2/B8CPFe-3 system of CpFeCHP(CH₃)B₈H₈ structures.

Fig. 5 The four lowest energy CpFeCHP(CH₃)B₉H₉ structures.

Fig. 6 The six lowest energy CpFeCHP(CH₃)B₅H₅ structures.

Fig. 7 The six lowest energy CpFeCHP(CH₃)B₆H₆ structures.

Fig. 8 The four lowest energy CpFeCHP(CH₃)B₇H₇ structures.

3. Results and discussion

3.1 The 11-vertex system CpFeCHP(CH₃)B₈H₈

The one experimentally known CpFeCHP(CH₃)B_n−3H_n−3 system is the 11-vertex system CpFeCHP(CH₃)B₈H₈ (Fig. 2).¹⁶ The experimental structure is derived from the most spherical closo 11-vertex deltahedron (Fig. 1) with the iron atom located at the unique degree 6 vertex and the phosphorus and carbon atoms located at the two degree 4 vertices. However, one of the Fe–B edges is lengthened to 2.636 Å relative to the ∼2.4 Å lengths of the other three Fe–B edges. Our theoretical study predicts three CpFeCHP(CH₃)B₈H₈ structures within 15 kcal mol⁻¹ of the global minimum B8CPFe-1 (Fig. 3 and Table 1). This global minimum does not correspond to the experimental CpFeCHP(CH₃)B₈H₈ structure since its phosphorus atom is located at a degree 5 rather than a degree 4 vertex.

Table 1 The three CpFeCHP(CH₃)B₈H₈ structures within 11 kcal mol⁻¹ of the lowest energy structure

Structure (symmetry)	ΔE kcal mol^−1	Vertex degrees			Heteroatom edges, Å	Polyhedron
		Fe	C	P
B8CPFe-1 (Cs)	0.0	6	4	5	Fe–P(2.327), Fe–C(1.980)	11-v deltahedron
B8CPFe-2 (C1)	3.1	5	4	4	Fe–P(2.146), Fe–C(1.912)	Fe⋯B(3.095) ⇒ f4
B8CPFe-3 (Cs)	3.5	6	4	4	Fe–P(2.158), Fe–C(1.960)	Fe–B(2.410)

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The next two closely energetically spaced CpFeCHP(CH₃)B₈H₈ structures B8CPFe-2 and B8CPFe-3, lying ∼3 kcal mol⁻¹ in energy above B8CPFe-1, are closely related to the experimental structure¹⁶ since in both of these structures both the phosphorus and carbon atoms are located at the degree 4 vertices. The Fe–B edges in the C_s structure B8CPFe-3 are not exceptional with two short ∼2.27 Å Fe–B edges and two longer ∼2.41 Å Fe–B edges. However, in B8CPFe-2 one of the Fe⋯B edges has lengthened to ∼3.09 Å (dashed line in Fig. 3) whereas the other Fe–B edges remain in the range 2.14 to 2.36 Å. The lengthening of one of the Fe⋯B edges in B8CPFe-2 has the effect of converting two adjacent triangular faces into a quadrilateral face and reducing the degree of the iron vertex from 6 to 5. This effectively converts the 11-vertex closo polyhedron into an 11-vertex isonido polyhedron with the same number of skeletal electrons.^23–25 The experimental CpFeCHP(CH₃)B₈H₈ structure lies between B8CPFe-2 and B8CPFe-3 with its elongated Fe⋯B edge of 2.636 Å. The experimental Fe–P and Fe–C distances of 2.134 and 1.955 Å, respectively, for CpFeCHP(CH₃)B₈H₈ (ref. 16) are close to the predicted values for B9CPFe-3 of 2.158 and 1.960 Å as well as those for B9CPFe-2 of 2.146 and 1.912 Å.

In order to provide additional insight regarding the experimental CpFeCHP(CH₃)B₈H₈ structure relative to the predicted structures B8CPFe-2 and B8CPFe-3, geometry optimizations starting from the X-ray coordinates of the experimental structure¹⁶ were performed. This led to a structure identical to B8CPFe-3 in both geometry and energy. In addition, scanning the fragile Fe–B bond in B8CPFe-2 to reach the same Fe–B bond length as in B8CPFe-3 provided an energy barrier of 0.77 kcal mol⁻¹ (Fig. 4). This low energy barrier suggests fluxional behavior between B8CPFe-2 and B8CPFe-3. In addition, the experimental ¹¹B NMR spectrum of CpFeCHP(CH₃)B₈H₈ is closer to the calculated ¹¹B NMR spectrum of B8CPFe-3 than to that of B8CPFe-2 (Table 2). This is consistent with the elongated Fe⋯B edge of length 2.636 Å being closer to the 2.410 Å value for B8CPFe-2 than to the 3.095 Å value for B8CPFe-2.

Table 2 Summary of ¹¹B NMR shifts. The numbering of the boron atoms (in parentheses) is relative to the X-ray crystal structure¹⁶

	^11B NMR chemical shifts in ppm
a Values taken from ref. 16.
B8CPFe measured	4.7(9), −1.9(8), −18.1(6,7), −22.4(4,5), −24.7(10,11)^a
B8CPFe-2 calculated	4.2(9), −4.5(7), −7.4(4), −9.9(8), −12.5(5), −18(11), −22.2(10), −33.7(6)
B8CPFe-3 calculated	3.4(9), −3.9(8), −15.9(6,7), −24.3(4,5), −28.7(10,11)

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3.2 The 12-vertex system CpFeCHP(CH₃)B₉H₉

The eight CpFeCHP(CH₃)B₉H₉ structures within ∼26 kcal mol⁻¹ of the lowest energy structure all have a central FeCPB₉ regular icosahedron and thus all degree 5 vertices (Fig. 5 and Table 3). The four lowest energy of these structures, lying within 10 kcal mol⁻¹ of the lowest energy structure B9CPFe-1, are the four possible structures with adjacent iron and phosphorus vertices (i.e., an Fe–P edge) and non-adjacent phosphorus and carbon vertices (no P–C edges). These four structures differ in the locations of the carbon vertices relative to the iron and phosphorus vertices. The four higher energy CpFeCHP(CH₃)B₉H₉ structures, at energies between 19.5 and 26.1 kcal mol⁻¹ above B9CPFe-1, have either non-adjacent iron and phosphorus atoms or adjacent phosphorus and carbon atoms.

Table 3 The four lowest energy CpFeCHP(CH₃)B₉H₉ structures within 19 kcal mol⁻¹ of the lowest energy structure

Structure (symmetry)	ΔE kcal mol^−1	Heteroatom edges, Å	HOMO–LUMO gap, eV
B9CPFe-1 (Cs)	0.0	Fe–P(2.101), Fe⋯C(3.151), P⋯C(3.357)	3.2
B9CPFe-2 (C1)	3.6	Fe–P(2.117), Fe–C(2.040), P⋯C(2.909)	3.0
B9CPFe-3 (C1)	6.2	Fe–P(2.092), Fe⋯C(3.158), P⋯C(2.960)	3.0
B9CPFe-4 (Cs)	9.4	Fe–P(2.082), Fe⋯C(3.693), P⋯C(2.905)	3.0

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3.3 CpFeCHP(CH₃)B_n−3H_n−3 (n = 8, 9, 10) systems

All six of the lowest energy 8-vertex CpFeCHP(CH₃)B₅H₅ structures (Fig. 6 and Table 4) have a central FeCPB₅ bisdisphenoid, which is the most spherical 8-vertex closo deltahedron (Fig. 1). The four lowest energy CpFeCHP(CH₃)B₅H₅ structures, lying within 4.8 kcal mol⁻¹ of B5CPFe-1, have the iron atoms at degree 5 vertices and Fe–P edges but differ in the location of the carbon vertex relative to the iron and phosphorus vertices. There is then a jump in relative energy to B5CPFe-5, lying 10.8 kcal mol⁻¹ above B5CPFe-1, which has the iron atom at a degree 5 vertex but not adjacent to the phosphorus atom (i.e., no Fe–P edge). The next CpFeCHP(CH₃)B₅H₅ structure B5CPFe-6, lying 12.5 kcal mol⁻¹ in energy above B5CPFe-1, has the iron atom at a degree 4 vertex. However, this iron atom is directly bonded to the phosphorus atom through an Fe–P edge.

Table 4 The six CpFeCHP(CH₃)B₅H₅ structures within 15 kcal mol⁻¹ of the lowest energy structure

Structure (symmetry)	ΔE kcal mol^−1	Vertex degrees			Heteroatom edges, Å	HOMO–LUMO gap, eV
		Fe	C	P
B5CPFe-1 (C1)	0.0	5	4	4	Fe–P(2.095), Fe–C(2.014)	2.4
B5CPFe-2 (C1)	0.0	5	4	5	Fe–P(2.222), Fe–C(1.999)	2.5
B5CPFe-3 (C1)	0.1	5	4	4	Fe–P (2.165)	2.7
B5CPFe-4 (C1)	4.8	5	4	4	Fe–P(2.174), Fe–C(1.940)	2.4
B5CPFe-5 (C1)	10.8	5	4	4	Fe–C(2.001)	2.9
B5CPFe-6 (C1)	12.5	4	4	4	Fe–P(2.038)	1.7

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The five lowest energy structures for the nine-vertex CpFeCHP(CH₃)B₆H₆ system, lying within 8.5 kcal mol⁻¹ of the lowest energy structure B6CPFe-1, are all based on the tricapped trigonal prism (Fig. 7 and Table 5), which is the most spherical 9-vertex closo deltahedron (Fig. 1). All five structures have the iron atom at a degree 5 vertex and an Fe–P edge. The four lowest energy structures have the carbon atom at a degree 4 vertex whereas the fifth CpFeCHP(CH₃)B₆H₆ structure B6CPFe-5 has the carbon atom at a degree 5 vertex.

Table 5 The six CpFeCHP(CH₃)B₆H₆ structures within 15 kcal mol⁻¹ of the lowest energy structure

Structure (symmetry)	ΔE kcal mol^−1	Vertex degrees			Heteroatom edges, Å	HOMO–LUMO gap, eV
		Fe	C	P
a This structure has a central FeCPB6 isocloso deltahedron.
B6CPFe-1 (Cs)	0.0	5	4	5	Fe–P(2.233)	2.0
B6CPFe-2 (C1)	1.5	5	4	5	Fe–P(2.176), Fe–C(1.947)	2.2
B6CPFe-3 (C1)	4.2	5	4	4	Fe–P(2.098)	2.0
B6CPFe-4 (C1)	7.7	5	4	4	Fe–P(2.097), Fe–C(1.940)	1.9
B6CPFe-5 (C1)	8.5	5	5	5	Fe–P(2.251), Fe–C(2.143)	2.4
B6CPFe-6 (C1)^a	13.2	6	4	5	Fe–P(2.770), Fe–C(1.937)	2.0

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The sixth CpFeCHP(CH₃)B₆H₆ structure B6CPFe-6, lying 13.2 kcal mol⁻¹ in energy above B6CPFe-1, is the only low-energy CpFeCHP(CH₃)B_n−3H_n−3 structure found in this work that is not based on one of the most spherical closo deltahedra in Fig. 1. Instead the structure of B6CPFe-6 is based a central 9-vertex isocloso deltahedron with the iron atom at the unique degree 6 vertex and the phosphorus atom at an adjacent degree 5 vertex (Fig. 7 and Table 5). However, the Fe⋯P edge is lengthened from the typical 2.0 to 2.3 Å distance to 2.770 Å in B6CPFe-6 (see dashed line in Fig. 7). Considering this lengthened Fe⋯P edge as no longer an edge leads to an isonido 9-vertex structure^23–25 having a quadrilateral face with a degree 5 iron vertex and a degree 4 phosphorus vertex. Such an isonido CpFeCHP(CH₃)B₆H₆ structure B6CPFe-6 is as consistent with its 20 skeletal electrons (= 2n + 2 for n = 9) as the closo tricapped trigonal prismatic structures for the five lowest energy CpFeCHP(CH₃)B₆H₆ structures. This lengthening of one of the Fe–P edges to the degree 6 iron vertex in the 9-vertex isocloso structure B6CPFe-6 is analogous to the lengthening of one of the Fe–B edges to the likewise degree 6 iron vertex in the 11-vertex closo structure B8CPFe-2 discussed above. Note that the 11-vertex closo deltahedron can also function as an 11-vertex isocloso deltahedron since it has a degree 6 vertex.

The three lowest energy structures in the 10-vertex system CpFeCHP(CH₃)B₇H₇, which lie within 2.4 kcal mol⁻¹ of the lowest energy structure B7CPFe-1, have the iron atom located at a degree 5 vertex, the phosphorus atom adjacent to the iron atom forming an Fe–P edge, and the carbon located at a degree 4 vertex (Fig. 8 and Table 6). The central FeCPB₇ polyhedron in these three structures is the bicapped square antiprism, which is the most spherical closo 10-vertex deltahedron (Fig. 1). After these three CpFeCHP(CH₃)B₇H₇ structures there is a large jump in energy to B7CPFe-4, which lies 14.7 kcal mol⁻¹ above B7CPFe-1. Structure B7CPFe-4 has the carbon atom as well as the iron and phosphorus atoms located at degree 5 vertices. Thus both of the degree 4 vertices in the central bicapped square antiprism of B7CPFe-4 are occupied by boron atoms.

Table 6 The four CpFeCHP(CH₃)B₇H₇ structures within 18 kcal mol⁻¹ of the lowest energy structure

Structure (symmetry)	ΔE kcal mol^−1	Vertex degrees			Heteroatom edges, Å	HOMO–LUMO gap, eV
		Fe	C	P
B7CPFe-1 (Cs)	0.0	5	4	5	Fe–P(2.073)	2.6
B7CPFe-2 (C1)	2.2	5	4	5	Fe–P(2.169)	2.6
B7CPFe-3 (C1)	2.4	5	4	5	Fe–P(2.162), Fe–C(1.949)	2.6
B7CPFe-4 (C1)	14.7	5	5	5	Fe–P(2.120), Fe–C(2.080)	2.8

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4. Conclusions

The central FeCPB_n−3 polyhedra in all of the lowest energy CpFeCHP(CH₃)B_n−3H_n−3 structures (n = 8 to 12) are the most spherical closo deltahedra (Fig. 1) as suggested by the Wade–Mingos rules for these 2n + 2 skeletal electron systems.^5–8 The heteroatoms in the FeCPB_n−3 polyhedra are so located to have adjacent iron and phosphorus atoms and non-adjacent phosphorus and carbon atoms in the lowest energy structures. In addition, the carbon atoms prefer energetically to be located at degree 4 rather than degree 5 or 6 vertices in accord with previous theoretical studies on metalladicarbaboranes²⁶ and metallatricarbaboranes.²⁷ The iron atoms prefer energetically to be located at the highest degree vertex, normally a degree 5 vertex except for the 11-vertex closo deltahedron, which has a degree 6 vertex for the iron atom.

The iron atom at the degree 6 vertex in the lowest energy 11-vertex CpFeCHP(CH₃)B₈H₈ structures is adjacent to the phosphorus and carbon atoms as well as four boron atoms. However, one of the Fe–B bonds in the 11-vertex CpFeCHP(CH₃)B₈H₈ structure appears to be fragile, readily elongating to ∼3.1 Å in one of the low-energy structures. This effectively converts the 11-vertex closo deltahedron having 18 triangular faces into a polyhedron having a single quadrilateral face as well as 16 triangular faces. This elongation of an Fe–B bond is also observed in the experimental CpFeCHP(CH₃)B₈H₈ structure,¹⁶ which has one elongated Fe–B deltahedral edge of ∼2.6 Å in addition to three normal Fe–B edges of 2.3 to 2.4 Å.

Acknowledgements

Funding from the Romanian Ministry of Education and Research, (Grant PN-II-RU-TE-2014-4-1197) and the U. S. National Science Foundation (Grant CHE-1057466) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Table S1A: initial CpFeCHPHB₅H₅ structures; Table S1B: distance table for the lowest-lying CpFeCHPCH₃B₅H₅ structures; Table S1C: energy ranking for all of the CpFeCHPHB₅H₅ structures; Table S2A: initial CpFeCHPHB₆H₆ structures; Table S2B: distance table for the lowest-lying CpFeCHPCH₃B₆H₆ structures; Table S2C: energy ranking for all of the CpFeCHPHB₆H₆ structures; Table S3A: initial CpFeCHPHB₇H₇ structures; Table S3B: distances table for the lowest-lying CpFeCHPCH₃B₇H₇ structures; Table S3C: energy ranking for all of the CpFeCHPHB₇H₇ structures; Table S4A: initial CpFeCHPHB₈H₈ structures; Table S4B: distance table for the lowest-lying CpFeCHPCH₃B₈H₈ structures; Table S4C: energy ranking for all of the CpFeCHPHB₈H₈ structures; Table S5A: initial CpFeCHPHB₉H₉ structures; Table S5B: distance table for the lowest-lying CpFeCHPCH₃B₉H₉ structures; Table S5C: energy ranking for all of the CpFeCHPHB₉H₉ structures; complete Gaussian09 reference (ref. 22). See DOI: 10.1039/c5ra17070b

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