Synthesis of closo- and nido- Metalloboranes from Metallocenes

formed as intermediates undergo relatively easy oxidative closure to a desired closo product.37 N i ~ k e l o c e n e , ~ ~ cobaltocene,3* and salts o...
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3758

Journal of the American Chemical Society

/

100:12

/ June 7, I978

Synthesis of closo- and nido- Metalloboranes from Metallocenes R. N. Leyden, B. P. Sullivan, R. T. Baker, and M. F. Hawthorne* Contribution No. 391 6 from the Department of Chemistry, Unicersity of California. Los Angeles, California 90024. Receiced Norember 2 , 1977

Abstract: Ten- and 12-vertex closo- and nido-metalloboranes are formed by nido-cage closure or polyhedral expansion of closo-borane anions employing metallocenes as the metal vertex source. The reaction of [(a5-C5Hj)2Ni] with nido(BllH13)*-, (B10H13)-, or (BgH12)- in the presence of a catalytic amount of N a / H g amalgam in glyme yields clos0-[(75CSH5Ni)BI I H I]-, ~ nido- [(a5-CsHsNi)BloH~z]-,and the isomeric closo- I - and [2-(aS-CsH5Ni)BgH9]- anions, respectively. The reaction of [(q5-CsHs)2Ni] with B I O H in I ~glyme yields nido-[( aS-C5H5Ni)BloH13],which can be reversibly deprotonatanion. The reaction of [(a5-C5H5)2Co] with nido-[($-C5HjNi)BloHl2]affords ed to give the nido-[(a5-CsHsNi)BloH12]the mixed bimetalloborane closo-[([a5-C5H5)2CoNi)B10Hl~]-. The reaction of [(q5-CjHj)2Ni] or [(a5-CjHsNiC0)2] with closo-(B11Hl1)~-,(BloHlo)*-, or (B9H9)*- in glyme produces closo-[(a5-C5H5Ni)BI,HI I]-, [(a5-CsHjNi)2BloHlo], and isomeric [ I - and [2-(05-C~HsNi)B9H9]-,respectively. The polyhedral expansion of borane anions involves the first example of the oxidative addition of a B-B bond array of a borane to a metal complex. The reaction of [2-(q5-CsH5Ni)BgHg]- with Clz in CH3CN yields the perchloro analogue [2-(a5-C5H5Ni)BgC1g]- which thermally isomerizes to [ 1-(s5-C5H~Ni)B9C19]-.The thermal and photolytic isomerization of closo- [ 1 - and [2-(qs-C5H~Ni)B9Hg]-and their perchloro analogues is also discussed.

Introduction Although considerable interest has been shown with regard to the synthesis and reactivity of nido-heteroboranes incorporating transition metals,’ the synthesis of closo-metalloboranes has yet to be fully explored. It is well known that closo-carboranes, C,B,H,+,, can be thought of as a formal replacement of m{BHl2+ vertices from the parent closo- borane dianions by m{CH)3+units2 Similar vertex replacement by (q5-C5H5Ni1V)3+(isoelectronic to {C-H)3+)3 or {v5(isoelectronic to {BH)2+)3moieties should yield isolable closo-metalloborane species with stabilities reflecting those of the closo-carboranes2 or borane d i a n i o n ~ . ~ Grimes et al.’1%12 have reported the preparation and characterization of two closo-cobaltaborane species, [ ( v 5 C ~ H ~ C O ) ~ and B~H [(q5-C5H5Co)3B3H5], ~] both of which contain so-called “extra” hydrogens. We report here the first preparation and characterization of closo-metalloboranes containing no “extra” hydrogens. In two preliminary communication^^^^ we have reported the synthesis of the icosahedral closo-nickelboranes [ l,2-(q5C5H5Ni)2B10HI0], [(q5-C5H5Ni)B1, H I11- and the isomeric ten-vertex square antiprismatic species, [ 1- and [ 2 - ( q 5 C5H5Ni)BgHg]-. In this paper we present the details of these results and of additional metalloboranes both closo- and nido-, synthesized by employing metallocenes as metal vertex sources. Results and Discussion Two general methods have been developed for the synthesis of nido- and closo-metalloboranes. The first method is termed nido-cage closure. The nido- borane species “closes” around the metal atom of the metallocene reagent with expulsion of a cyclopentadienyl moiety. This process may occur with or without loss of molecular hydrogen or possibly by hydride transfer to the cyclopentadienyl ligand and has many precedents in metallocarborane syntheses.’ The second route to closo-metalloboranes may be classified as polyhedral expansion of closo-borane anions. In this case a metallocene (i.e., [(v5-CjH5)2Ni]) reacts with a closo-borane anion of n vertices to give a higher homologue of n 1 or n 2 vertices. This process differs from the reductive polyhedral expansion of carboranes or m e t a l l o ~ a r b o r a n e sand ~ , ~is the first example of the oxidative addition of a B-B bond array in a closo-borane to form a closo-metalloborane.

+

+

0002-7863/78/1500-3758$01 .OO/O

closo-[(q5-C5H5Ni)B11H11](I) from nido-(B11H13)~-by Nido-Cage Closure. Solutions of (BIlH13)2- and nickelocene in acetonitrile remain unchanged indefinitely at room temperature. Addition of a catalytic amount (less than 20% of 1 equiv) of sodium amalgam quickly gives a dark brown solution with evolution of hydrogen. Removal of the acetonitrile and treatment of a n aqueous solution of the product mixture with (CH3)4NCI yields 63% of the bright yellow I after recrystallization. The 60-MHz ‘H N M R spectrum of I consists of two sharp singlets of relative area 5: 12 (Table I) which are assigned to the cyclopentadienyl and the tetramethylammonium resonances, respectively. The 80.5-MHz IlB N M R spectrum consists of three sharp doublets of relative area 1:5:5 (Table 11). This is distinctive for a CsCicosahedral structure with one heteroatom vertex. The complex I is assigned the structure depicted in Figure 1, which is consistent with these data. It is thus considered to be a formal analogue of (CBl1H12)-.l0 Upon collection of the gas evolved during the reaction it was found that slightly more than 1 mol of H2 was produced for each mol of I isolated. This reaction is also found to proceed at elevated temperatures in a nonreactive solvent in the absence of sodium amalgam. A solution of ( ( C ~ H ~ ) ~ N 1H13) ) ~ ( Band I nickelocene in 1,2-dimethoxyethane reacts a t reflux temperature in several hours. A 52% yield of I as the ((C4H9)4N)+salt is isolated and again the hydrogen gas is found to be slightly in excess of 1 mol for each mol of I isolated. nido-[(q5-C5H5Ni)B1oH13] (11) and nido-[(q5-C5H5Ni)BloH121- (111) from B I O H Ior~ (BloH13)-. When nickelocene is added to a solution of decaborane in 1,2-dimethoxyethane or benzene an immediate exothermic reaction ensues. If the reaction is quenched in less than 1 h a 60% yield of a neutral red-brown complex, 11, can be isolated. Treatment of I1 with aqueous base followed by (CH3)dNCI results in precipitation of the anionic red complex, 111. Significant amounts of [(B1oH12)2Nil2- are isolated from the initial reaction mixture and at long reaction times this becomes the dominant product. This latter product has also been shown to arise from the interaction of simple Nil1 salts with either ( B I o H ~ ~or) - ~ ~ (B 1 OH 1 4)2-.14 The 100-MHz ‘H N M R spectrum of I1 consists of a sharp singlet assigned to the cyclopentadienyl resonance and a broad resonance centered a t 7 10.5 assigned to B-H-B bridging 0 1978 American Chemical Society

Hawthorne et al. / closo- and nido-Metalloboranes from Metallocenes Table I. ' H N M R Chemical Shifts No.

( T ) ~

Cornpd

(ICH~)~N)[(CSH~N~)BIIHIII

I I1 Ill IV

3759

[(CSH~N~)BIOHI~I ((CH314N)[ ( C S H ~ N ~ ) B I O H I ~ I ((C6Hd4AS)[(CsH5)2NiCoBioHiol

Solvent

(C5Hs)- (area)

Cation (area)

CD3CN Acetone-ds CD3CN CD3CN

4.62 (5)

6.95 ( 1 2)

4.92 ( 5 ) 4.83 ( 5 ) 4.58 (5)

6.95 ( 1 2) 2.30 (20)

b

5.03 (5)

[(C~H~N~)ZBIOHIOI (ICH314N [ 2 - ( C s H s N i ) B 9 b I (ICH314W [ 1 -(GHsNi)B9H91 (ICH3hN) [2-(CsHsNi)B9ChI

V VI VI1 VI11

Acetone-ds CD3CK CD3Ch CD3CN CD3CN

IX Taken at 100 M H z with Me4Si internal standard. with coincidental overlap. (I

Also T 7.55 (-10) assigned to cage B-H,

4.54 4.89 (5) 4.30 (5) 4.36 (5)

4.40 (5) 7

6.95 6.95 6.95 6.95

( I 2) ( 1 2) (12)

( I 2)

8.31 (-2.5) assigned to bridge hydrogens

Table 11. 80.5-MHz "B N M R Spectral Data for the Metalloboranes No.

Cornpd

I I1 Ill

Chemical shiftU (re1 intensity)

((CH~)~N)[(CSHSN~)BIIHII~f 1 9 . 9 ( I ) , +16.0 (5), -5.1 (5) +4.81 ( I ) , $5.8 ( I ) , -6.7 (3), -26.9 (2) $6.4 (2). -6.0 (3), -25.8 (2) +19.8 ( I ) . +10.7 (3), f 7 . 9 (2). -5.2 (2) f 2 6 . 1 (2)- $21.3 (4). +6.7 (2) +12.6 ( I ) . +6.9 (3). -14.7 (2). -21.8 (2) f 2 9 . 0 (4),- I .3 (4) ( { C H ~ J ~ N ) [ ~ - ( C ~ H S N ~ ) B ~ C ~ ~+ ~6 0 . 5 ( l ) , + 1 8 . 2 ( 1 ) , + 1 2 . 4 ( 3 ) , - 0 . 6 ( 2 ) , - 1 1 . 1 ( 2 ) (ICH314N) [ 1 - ( C ~ H S W B ~ C ~ I +53.4 ( I ) . $38.5 (4). $18.4 (4)

+10.5 (3), ((CH~)~N)[(CSHSN~)BIOHI~I + I 1.8 (3), (2). ( ~ C ~ H ~ ~ ~ A S ) [ ( C S H ~ ) ~ C O N ~ B I O+45.7 HIO~ [(CSHW~BIOHIOI $54.3 (2), ({CH,)~N)[Z-(CSHSN~)B~H~I +59.0 ( I ) , (lCH314N) [ 1 - ( C ~ H S N ~ ) B ~ H ~ I $78.6 ( I ) .

[(CSH~N~)BIOHI~I

IV V VI VI1 VI11

IX

Solvent CH3CN CH3CN CH3CN CH3CN Glyme CH3CN CH3CN CH3CN CH3CN

With BF3-EtlO as external standard.

Table 111. Cyclic Voltammetric DataU E112

NO.

t5.1 (1281

1 11

Ill IV V

(1)

(5)

Figure 1. The structure and

VI VI1 VI11 IX

Compd

Oxidn

Redn

({CH3)4N)[(C5H5Ni)BI] H II ] + I .70e -1 .50C [(CsHsNi)BIoH131 +0.79e ((CH3)4N)[(C5HSNi)BloH 1 2 1 $0.61 ( { C ~ H & A S ) [ ( C ~ H J ) ~ C O N ~ $0.45' -1.55h.d BioHiol $1 .30d +1.73e -0.27' [(CsHsNi)zB~oH~ol

- 1.35'

( ( C H ~ ) ~ N ) [ ~ - ( C ~ H S N ~ ) +0.87C B ~ H ~ ]-1.42 ({CH3}4N)[1-(C=,HsNi)B9H9] +0.77P - I .52 ( { C H , ) ~ N ) [ ~ - ( C ~ H J N ~ ) B ~ C I ~ ] -0.40' ((CH&N) [ 1 -(C~H5Ni)BgC191 -0.35'

(5) IlB

Determined in acetonitrile with 0.1 M tetraethylammonium NMR spectrum of [ ( C ~ H ~ N ~ ) B I I H I I ] -perchlorate as supporting electrolyte, platinum button electrode. Potentials vs. SCE. Hanging mercury drop electrode. Reversible. d Quasi-reversible. Irreversible.

protons. The 60-MHz IH N M R spectrum of 111 exhibits two sharp singlets assigned to the cyclopentadienyl and tetramethylammonium resonances, respectively. The 80-MHz IB N M R spectra of both I1 and 111 have a qualitatively similar appearance. The spectrum of I1 consists of four doublets of relative area 3:2:3:2 with the two high-field doublets partially collapsed due to multiple proton coupling. The spectrum of 111 also has four doublets of relative area 3:2:3:2. The chemical shifts of both compounds listed in Table I1 are very close to those of [Ni(B10H12)2I2-,I3 which exhibits resonances of relative area 4:1:1:2:2. All three can be interpreted as arising from a 2:2:2:2:1:1 pattern with coincidental overlap as required for a n 1 1-atom icosahedral fragment of pseudo-C5, symmetry (Figure 2). The charged species, 111, is formulated as nido-[(t15-C~H5Ni)BloH12]-. The species I1 is then the corresponding protonated product, nido- [(q5-CsHsNi)B I o H I ~ This ] . formulation is supported by the fact that I1 can be deprotonated to 111 by aqueous base and I11 can be converted to I1 by treatment with concentrated aqueous HCI in ether.

The species 111 is made in slightly lower yield by treatment of nickelocene with nido-(BloH13)-. Again prolonged reaction

times lead to [(BloH12)2NiI2-. This is analogous to the synthesis reported by MuettertiesI3 (vide supra). Reductive or basic conditions promote the rapid conversion of 111 to the bis complex. The cesium salt of 111 appears to be completely stable to 180 OC under high vacuum. Pyrolysis of 111 for 1 h a t 230 OC results in loss of a small amount of hydrogen but the residue is a mixture of insoluble materials and unreacted 111. Aqueous solutions of 111 do not immediately react with H202 despite the quasi-reversible oxidation wave a t +0.16 V (vs. SCE) seen in the cyclic voltammogram (Table 111). c ~ ~ ~ ~ - [ ( ( ~ ~ - C ~ H(IV) ~ ~from Z C ~nido-[(qsN~)B~~H~~] CSH~N~)B~O (111). H ~Treatment Z] of a solution of cobaltocene and 111 in 1,2-dimethoxyethane with sodium amalgam results in a mixture from which the green anionic complex, IV, can be isolated as the tetraphenylarsonium salt in 2% yield. An

3760

Journal of the American Chemical Society

/ 1OO:I 2 / June 7 , 1978

L

w,

m

II

0 CH r'\ 4 BH

Figure 2. Proposed structure of [(C5H5Tii)B10H13] and [ ( C ~ H ~ N ~ ) B I Oand H ~their ~ ] - acid-base chemistry. Table IV. Elemental Analyses for the Metalloboranes Compd 1

Calcd Found Calcd Found Calcd Found Calcd Found Calcd Found Calcd Found Calcd Found Calcd Found

II 111

IV

v VI VI1

IX

%C

%H

32.96 33.15 24.5 I 24.40 33.98 34.16 54.50 54.46 32.83 32.62 35.52 35.38 35.52 35.72 17.60 17.60

8.61 8.58 7.41 7.22 9.19 9.25 5.38 5.22 5.52 5.44 8.61 8.51 8.61 8.96 2.79 2.75

% As

10.00 10.15

%

B O

% CI

36.26 35.90 44.12 43.93 33.98 33.96 14.40 14.20 29.55 29.88 31.97 29.12 15.84 15.40

%c o

%%

96 Ni5

4.27 4.27

17.90 17.52 23.96 23.1 1

4.40 4.39 7.86 7.57

4.60 4.70

18.45

18.48 7.83 7.89 32.10 3 1.48 19.29 18.09

5 1.93 5 1 .SO

Elemental analyses of the nickelaboranes were consistently found to have low percentages for boron and nickel presumably owing to the formation of nickel borides during combustion.

m

Jp

Figure 3. Synthesis and structure of [(CsH~)zCoNiBioHio]-.

alternate method involving addition of 111 to a mixture of CoC12, NaC5H5, and excess N a H produces IV in 10% yield. The IH N M R spectrum of IV exhibits a broad singlet due to the tetraphenylarsonium cation and two cyclopentadienyl resonances of relative intensities 20:5:5.The IlB N M R spectrum consists of five doublets of relative intensities 2:1:3:2:2. The elemental analysis is consistent with the formulation ( { C ~ H ~ I ~[ (A{q5-C5H5}2CoNi)BloH S) I 01 (Table IV). Based upon these data the gross structure of IV is assigned as icosahedral with a formal cyclopentadienyl NilV moiety adjacent to a formal cyclopentadienyl C O * ~ )see ' Figure 3). Thus the borane fragment of IV is formally the arachno-(BloHlo)6ligand and the entire complex is isoelectronic with the previously reported [ 1,2-(.r75-C5H5Ni)zB1~Hl~] (vide infra). The similarities of these bimetallic species are apparent upon examination of their IlB N M R spectra (Figure 4). The IlB chemical shift positions and overall appearances are similar, the main difference being symmetry lowering in the Co-Ni species.

The sodium salt of IV is quite stable in neutral or slightly basic aqeuous solutions. Acidic p H conditions cause decomposition with the deposition of a white solid. Strongly reducing solutions also appear to promote decomposition. This observation may explain the low yield of this compound. closo-[2-(q5-CsH5Ni)B9H~]-(VI)and [I-(q5-C&Ni)B9H9](VII) from (nido-BgH~$. Treatment of a solution of ( { C H ~ ) ~ N ) ( B ~and H Inickelocene ~) in acetonitrile with 1 equiv of sodium amalgam produces a rapid color change from dark green to dark red with noticeable evolution of gas. The red and yellow anionic complexes, VI and VII, can be isolated as ((CH3)4N)+salts by column chromatography on alumina in 18 and 7% yields, respectively. The yield of VI1 is increased at the expense of VI if care is not taken to minimize the exposure of solutions to heat, light, and acidic conditions. The 80-MHz 'B N M R spectrum of VI exhibits five doublets of relative area 1:1:3:2:2 and the spectrum of VI1 consists of three doublets of relative area 1 :4:4. The IH NMR spectrum of both compounds consists of two singlets of relative area 5:12 due to the cyclopentadienyl and the tetramethylammonium resonances, respectively. The 1:1:3:2:2 pattern of doublets which is observed for compound VI in its IlB spectrum is consistent with a bicapped Archimedean square antiprism with the (CSHSNi] moiety in the equatorial position if there is coincidental overlap of a doublet of area one with a doublet of area two. The 1:4:4 pattern of doublets observed for compound VI1 is unique for the highly symmetrical bicapped square antiprism with the nickel atom in the apical position (Figures 5 and 6 ) . The IlB chemical shift of the apical boron atom in VI1 appears at very low field (-74 ppm). This is another example of

Hawthorne et al.

/

closo- a n d nido-Metalloboranes from Metallocenes

3761

'H decoupled 80.5 MHz

+

'b NMR (in C H S N )

1,-

-0.7

4

Heot a L q h t

80.5 MHz "B NMR (in 1.2 Dimthoxyethone) -a3

b.

IY)

h

Bo%-

+

Ni(C#&

Wl$li2(csY2

Figure 4. "B N M R spectra of the icosahedral bimetallocarboranes: (a) (~C~H~~~A~)[(CSHS)~CON~BIOHIOI; (b) ( C S H S N ~ ) ~ B I O H I O . -29WI

a) -TIMIWl

2-

i

(C5H5NiC0)2 c

w2b)

I'

h 0 CH Ow

Figure 7. Formation of [ ( C ~ H ~ N ~ ) ~ B Ivia O Hpolyhedral IO] expansion of (BioHio)*-.

Ill

Figure 5. "B NMR spectra of [(CsH5Ni)BgH9]-: (a) the I isomer; (b) the 2 isomer.

the "antipodal effect", i.e., low-field shifts (relative to other shifts occurring in that cluster) which occur at the boron atom placed symmetrically across from a h e t e r o a t 0 m . l ~ , ~ 6 , ~ ~ Formation of Metalloboranes through Nido-Cage Closure. The reaction of nickelocene with ( B I ~ H I ~or) ~(BsH12)appears to depend more upon the electronic properties of the metallocene18 rather than the nature of the borane. Under the conditions used in the nickelocene reactions both cobaltocene and ferrocene fail to produce the corresponding closo-metalloboranes. This result may be due to the decreased lability of the cyclopentadienyl ligand (going from Ni Co Fe)19320or the decreased stabilitv of the exDected closometalloboranes (Le., [(v5-C5H5do)B~ IHI or [($C5H5Fe)B1~H I ]~~ - ) . ~ ] Polyhedral Expansion of Borane Anions. [(q5-C5H5Ni)BIIHIII- from (B11HII)2-. Heating a solution of the closo-

-

+ Na/w

Figure 6. Synthesis and structure of [(CsHsNi)B9H9]- isomers.

-

19-

I ) ~ - as , the tetrabutylammonium salt, borane anion, (BI and [(v5-C5H5NiC0)2] in tetrahydrofuran for several hours gives a mixture from which I (as the tetrabutylammonium salt) can be isolated in 23%.yield by chromatography. I is also formed in good yield by heating ( ( C ~ H ~ ) ~ N ) Z ( B I with IHII) nickelocene in 1,2-dimethoxyethane. The very reactive nickel reagent2* [ (C5H5)3Ni~](BF4) reacts immediately with (B11Hl in tetrahydrofuran. The resulting mixture of products in this case is quite complex and I is produced in only low yields (ca. 5%), thus making the former two methods preparatively more suitable. closo-[1,2-(r15-CsH5Ni)zBloHloj(V) from closo-(BloHl~)~-. The reaction between closo-(BloH~o)~and [(v5-C5H5)2Ni] or [(vs-C5H5NiC0)2] in 1,2-dimethoxyethanea t 25 OC rapidly produces a dark brown solution and a golden brown precipitate, V. Optimum reaction conditions are achieved with the tetraethyl- or tetrabutylammonium salts for solubility reasons. The IlB N M R spectrum obtained with this species reveals a 1:1:2:1 pattern. This is the expected symmetry splitting of either an octadecahedron with a cyclopentadienyl nickel moiety occupying the seven-coordinate vertex or a bimetallic nickel analogue of 0-or m-carborane (Figure 7). Mass spectral

Journal ofthe American Chemical Society

3162

data show a parent peak with a cutoff at m/e 36323and elemental analysis reveals a Ni:B ratio of 1:5. The structural assignment was thus definitive for a bimetallic analogue of o- or m-carborane, [ 1 , 2 - ( a 5 - C ~ H ~ N i ) z B ~ o(or H lmeta-l,7). ~] The IH N M R spectrum shows only one cyclopentadienyl resonance which is also consistent with the structural assignment. Support for the ortho structural assignmentz4 for V comes from the similarity between the IIB spectra of IV and V. The direct formation of product without an oxidation step and the use of a 2:l nickel reagent to (BloH10)~-ratio (with yields based on nickel of greater than 50%) substantiate the reaction stoichiometry shown in eq 1 and 2.

/

100:12

/ June 7, I978

These compounds parallel the chemistry and possess the stability of their parent claso-boranes. The nido-metalloborane; [$-C5H5Ni)BloHI-J-, appears to be even more hydrolytically stable than its borane analogue (BI I H I ~ ) ~ - . Compound I comes closest to having the stability of its parent, ( B I ~ H I ~ )or~its - , monocarbon analogue ( C B I I H I ~ ) - . As an example, the nickel complex can conveniently be separated from less stable impurities by treatment with an aqueous solution of H202 which oxidizes the impurities to borate and nickel oxide. Attempts to perform a polyhedral contraction reaction by heating I in ethylene glycol/H2O/EtOH-KOH for several days resulted in the recovery of unreacted I. The hydronium salt of I was prepared by passing a solution of the 2[(vS-C5HsNiC0)21 + (BloHlo)2tetramethylammonium salt through a hydrogen-form cation closo- [ ~ , ~ - ( . ~ ~ ~ - C S H S N ~ ) ~ B I O H I O ]exchange column. This salt is a strong acid and remains un2CO 2[q5-C5HsNiCO]- ( 1 ) changed on removal of solvent at room temperature. Heating the residue under vacuum, however, results in decomposition 2[(q5-C5H5)2Ni] (BloH I O ) ? a t less than 100 OC. closo-[l,2-(s5-C~H~Ni)2B~oH~o] + ~ ( C S H S ) - (2) Treatment of I with DCI in acetonitrile/D20 a t room temperature over several days results in partial deuteration of the Despite variations in the experimental procedure, no evipolyhedral boron atoms as evidenced by the infrared spectrum. dence could be found for the monometallic intermediate. This The l l B N M R spectrum exhibited a partial collapse of the is not too surprising considering that the cyclopentadienylnickel(1V) moiety prefers low-coordinate vertex positions26 low-field area one doublet and the highest field area five and that the cage opening reaction is probably initiated by the doublet while the remaining doublet was unchanged. {$-CSHSNiJ vertex environment. Thus the monometallic ocExhaustive chlorination of I produces a mixture of species tadecahedron might be expected to be an unstable species or which are only partially chlorinated. Reaction of I with IC1 a t reactive intermediate relative to the bimetallic icosahedral 0 " C in acetonitrile produced the monoiodinated derivative. product. The IIB N M R spectrum indicated that the product was a Polyhedral Expansion of (B9H9)2-. Isomers of c/oso-[(~~- mixture of isomers and the IH N M R spectrum consisted of CsHsNi)B9H9- from ~/oso-(BgH9)~-.The reaction of [($three closely spaced cyclopentadienyl resonances in the ratio 5:5: 1. This suggests a statistical distribution of isomers. C S H ~ N ~ C Owith ) ~ ]Cs2BgHg in acetonitrile solution a t room temperature gives both [ l - and [2-(q5-C~H5Ni)B9Hg]-,VI1 Compound VI can be successfully perchlorinated by treatand VI (vide infra), and did not proceed to form the bimetallic ment with Clz in acetonitrile to yield the bright green [2octadecahedron. This is generally consistent with the high (qS-C5H5Ni)B9C19]- (VIII). Compound viii exhibits five stability of the ten particle square antiprism and the aversion singlets in the IlB N M R spectrum of relative areas 1:1:3:2:2. While the chemical shifts are slightly different, the splitting of cyclopentadienylnickel(1V) moiety for very high coordinate pattern is very similar to that of VI. Attempts to chlorinate VI1 positions on the polyhedral surface.26 with Cl2 resulted in a bright red, polar compound which could Formation of closo-Metalloboranes via Polyhedral Expansion of doso-Borane Anions. The first polyhedral expansion not be purified in the usual manner by chromatography. However, the deep yellow [ 1-(q5-C5H~Ni)BgC19]-(IX) is reactions of carboranes required an initial two-electron reduction of the polyhedron. Numerous examples of the reducobtained by heating VI11 for several minutes a t 130 O C . tive polyhedral expansion of carboranes and heteroatom carThe mixed metal bimetallic, IV, is the least stable of the boranes27 have now established the generality of this method. closo-metalloboranes. As an example, deuteration studies Recently, Stone and co-workers have demonstrated that lowunder acidic conditions were impossible to conduct owing to valent platinum reagents are capable of direct insertion into the instability of IV under acidic conditions. closo-carborane cages, thereby by-passing an explicit reducIsomerization of Ten-Vertex Nickeloboranes. In a sealed tube, an acetonitrile solution of VI completely isomerizes to tion step.28,29Grimes et al. have successfully effected at higher VI1 after several hours a t 130 OC. However, in 0.1 M trifluotemperatures direct insertion of [ Fe(CO)s] and [ q 5 roacetic acid in 90% CHjCN/lO% H2O in a sealed evacuated CsH&o(C0)2] .30 Both of these processes bear a formal tube, the isomerization is essentially complete in much less than similarity to the oxidative-addition reaction. Attempts to extend this oxidative-addition reaction to the borane anions using 1 h at 80 OC. The isomerization reaction therefore appears to be acid catalyzed. The isomerization of the perchloro species [cis-(PPh3)2PtC12] in alcoholic chloroform resulted in the VI11 to IX appears to proceed much more rapidly, showing production of an ethoxy-substituted nido-platinaborane (Pt") only IX in the l i B N M R spectrum after several minutes at 130 instead of the desired closo product (Pt1V).31 O C . The isomerization of VI11 is relatively insensitive to The reaction of nickelocene with the closo-borane anions acid. appears, then, to be the first example of the oxidative addition of a boron-boron bond array in a closo-boron hydride. The The isomerization of the (C5H5Ni)moiety from the equatorial position in VI to the apical position in VI1 can also be nucleophilic nickelocene molecule (a formally 20-electron achieved photochemically. This is the first reported polyhedral complex possessing two unpaired electrons) can be conceptually thought of as donating two electrons to effect the cage photochemistry of a pure metalloborane species.32Irradiation opening, followed by cyclopentadienyl ligand loss and oxidative from a low-pressure Hg lamp through Pyrex cleanly converted addition of the borane moiety to yield the closo-nickelaborane VI to VI1 in acetonitrile solution while prolonged irradiation ( N i I"). of VI1 caused no change. Chemistry of the Polyhedral Metalloboranes. Compounds Photolysis of IX reversed the thermal isomerization to I-IX are all air and water stable. With the exception of VI and produce a mixture of VI11 and IX on irradiation with a tungVIII, which isomerize to VI1 and IX, respectively, they also sten iodide lamp. The isomerization was so sensitive to light exhibit high thermal stability. The neutral bimetallic V can that purification of VI11 was complicated by exposure to be heated to 350 "C and subsequently sublimed a t 265 O C in normal room light. Irradiation of the green complex VI11 also vacuo over a period of hours with only partial decomposition. produced a mixture of VI11 and IX.

-

-

+

+

+

Hawthorne et al.

/ closo- and nido-Metalloboranes from Metallocenes

Table V. Infrared Spectra for the Metalloboranesa NO.

I II Ill

IV

V

VI

VI1

VI11

IX

a

Compd

Absorptions, cm-l (intensity)

2490 (s), 1020 (s), 950 (s), ((CH3)4N)[CjH~lc'i910 ( M ( = -29 (m), 728 BIIHIII (m) 2515 (s), 1415 (m), 1175 (m), [ $-CsHsN i B I OHI 31 1000 (m) 990 (s), 880 (m), 840 (w), 745 (m) 2490 (s), 1180 (m), 1040 (m), ({CH&K)[CjH5Ni995 (s), 950 (s), 885 (m), BioH I 21 843 (w), 827 (m), 742 (m), 725 (m) ( ( C ~ H ~ } ~ . A S ) [ ( C ~ H S ) ~2490 C O -( S ) , 1600 (m), 1080 ( S ) , 1000 (m), 820 (m), 740 (s), Y~BIOHIOI 690 (m) 2500 (s), 1408 (m), 1015 (s), [1,2-(CjHjYi)2BloHlo] 905 (m), 890 ( m ) , 850 (s), 840 (s), 745 (m) ( ( C H 3 ) 4 ~ ) [ 2 - ( C s H s N i ) - 2490 (s), 1360 (w), 1290 (w), 1230 (w), 1050 (m), 1020 BsHsl (m), 970 (m), 950 (s), 905 (51, 820 (SI, 790 (m) ( ( C H ~ J ? N ) [ ~ - ( C ~ H S I2500 ~ ) - (s), 1170 (w),1020 (w), 980 (m), 950 (s), 910 (s), BsHsl 825 (s), 730 (s) (s), 1050 (s), 950 (s), 883 ( { C H & N ) [ ~ - ( C S H ~ N ~ )1140 (m), 841 (m), 833 (m), 786 BsCb1 (SI, 700 (s) ( l C H , } ? ~ ) [ l - ( C j H S U i ) - 1720 (w), 1240 (w), 1190 (w), BsCh1 11IO(m), 1090(m), 1020 (m), 1000 (s), 895 (s), 848 (w), 828 (m), 740 (SI

Taken in Nujol mull.

s = strong. m = medium, w = weak.

This observation has no known counterpart in boron chemistry; although thermally reversible photochemical isomerizations have been r e p ~ r t e d ,the ~ ~steady , ~ ~ state ratio of isomers always favors one isomer exclusively. In the Bg nickelaboranes a 1 :2 equatorial/apical mixture is observed and is accessible photochemically from either isomer. W e intend to study this photochemical rearrangement in some detail. Infrared and Electronic Spectral Data. The infrared spectra (Table V) of all the complexes described above exhibited bands in the region 2500-2600 cm-' due to terminal B-H stretching vibrations. The spectra of complexes I-IX also contained a band of medium intensity in the region 810-840 cm-1 assigned to the (C5H5)- ligand. In addition, the spectra of complexes I, 111, and VI-IX contained a band a t 950 cm-I due to the ((CH&N)+ cation. W e were unable to detect bands in the region 1600-1900 cm-I in the spectra of I1 and 111 which might be assigned to BHB bridge groups. This is not unusual, however, as bands of this type are usually very weak.45 The electronic spectral data (Table VI) were generally of little use in providing structural information and no attempt was made to assign the bands to individual electronic transitions. The electronic spectra did, however, prove to be a convenient means of monitoring the thermal isomerization of VI VI1 and VI11 IX. Electrochemistry of the Metalloboranes. The values of the oxidation and reduction potentials for the cfoso-metalloboranes are listed in Table 111. The qualitative features of the cyclic voltammograms for each of the closo compounds are very similar. The reduction waves are all one-electron reversible reductions and the oxidations are all irreversible for the nickelaboranes. The bimetallic compound, V, exhibits two one-electron reductions which is consistent with the observations for bimetallic m e t a l l o c a r b o r a n e ~The . ~ ~ cobalt-nickel compound in addition exhibits a quasi-reversible oxidation wave which can be assigned to the formal couple.

-

-

3763

Table VI. Electronic Soectra (Acetonitrile Solution) No.

Compd

, , , A,

nm (log e )

I ((CH3)4N)[(CsH5Ni)271 (4.72) BiiHiil 111 ( ( C H ~ J ~ N ) [ ( C S H ~ N ~520 ) - sh (1.62), 365 (3.20), 290 (4.33). 266 sh (4.28) BioH I 21 IV ( { C ~ H S I ~ A S ) [ ( C S H S ) ~ 308 C O -(4.42), 270 (4.08), 262 BioHioI (4.1 I ) , 240 (4.36) VI ( { C H , ) ~ K ) [ ~ - ( C S H ~ N 518 ~ ) - (2.69), 329 (3.72), 263 BsHsl (4.36) VI1 ({CH3/4N)[l-(CSHjNi)349 sh (3.18), 283 (4.73) BsHsl VI11 ({CH&N)[2-(C,H5Ni)664 (2.77). 438 (3.41). 355 BsCh1 (4.15). 312 (4.15), 269 (4.02) IX ({CH314N)[l-(C5H51\'i)- 415 (3.19), 329 (4.59), 258 BsCb1 (3.98). 213 13.86)

By analogy with the NilV carborane corn pound^^^,^^ the first reduction wave seen in the cyclic voltammograms for the closo-nickelaboranes can be assigned to the NilV-Nil1' couple. The reduction potential for [(s5-C5H5Ni)B1 ] H I11- is very close to that of the icosahedral monocarbon carborane nickel complex, [1,2-(a5-C5H5Ni)CBloHll] (-1.57 V).25The icosahedral binickelaborane, V, also has a cyclic voltammogram that bears a resemblance to [ l,2-(.rl5-C5H5Ni)CB10Hl I ] and suggests the formal similarity of a cyclopentadienylnickel(1V) moiety to a (CHI3+group on the icosahedral surface.25 Chlorination of the ten-vertex nickel complex shifted the reduction potential by more than 1 V, thus making the nickel atom much more easily reduced. Adding the electron-withdrawing chlorine substituents onto the cage would be expected to decrease the donor ability of the borane ligand thus decreasing the effective electron density around the nickel atom. The nido-metalloborane, [($-C5H5Ni)BloH~*I-, exhibits only a single quasi-reversible oxidation wave at 0.61 V. This may be due to the formal NiI1-Ni1I' couple.

Experimental Section Physical Measurements. The 60-MHz IH N M R spectra were obtained using the T-60 Varian instrument, and the 80.5-MHz IlB spectra were recorded using the cryogenic spectrometer designed by Professor F. A. L. Anet and Dr. C . H. Bradley. Infrared spectra were determined using a Perkin-Elmer Model 137 spectrometer (Table V). Electronic spectra were obtained on the Cary 15 and Beckman Model DB spectrometers (Table VI). Cyclic voltammetry was performed using an instrument based on the design by H a ~ l e y Elemental .~~ analyses were performed by Schwarzkopf Microanalytical Laboratories, Galbraith Laboratories, and the UCLA microanalysts. Materials and Procedures. Unless otherwise stated, reactions were performed in a nitrogen atmosphere using reagent grade solvents which had been previously saturated with nitrogen. Workup was carried out in the presence of oxygen. however, since many nido species formed as intermediates undergo relatively easy oxidative closure to a desired closo product.37 N i ~ k e l o c e n e cobaltocene,3* ,~~ and salts of ~ l o s o - ( B l l H 1 1 ) ~ - -, ~( B ~ l o H l ~ ) ~ -and , ~ ~ -(B9H9)2-,41.42 nido( B ~ H I ~ )and - , ~( B~ l l H 1 3 ) ~ -39 were prepared by published procedures. Cyclopentadienylnickel carbonyl dimer was used as received from Alfa-Ventron Carp. Decaborane was obtained from commerical sources and sublimed at 80 OC under high vacuum. All solvents were distilled under N2 from an appropriate drying agent as described by ~01iy.44 ( { C H ~ J ~ N ) [ ~ ~ ~ - - C S H ~ N(I)~ from J B , I(B1lH13J2-. H,,] Sodium amalgam ( 1 g. 0.48%, 2. I mequiv) was added to 2.0 g ( 1 1 mmol) of Na2Bl ,HI3 and 2.1 g (1 1 mmol) of nickelocene in 200 mL of acetonitrile. The mixture was stirred for 12 h, the solvent was removed under reduced pressure. and 150 mL of water was added. The solution was carefully treated with H2Oz until the vigorous reaction had ceased. The solution was filtered through Celite and excess tetramethylammonium chloride was added. The dark yellow precipitate

3164

Journal of the American Chemical Society

was isolated by filtration and recrystallized from acetone/water to give 2.4 g (63%) of bright yellow needles of I. Similar yields were obtained without the addition of H202 but required more lengthy purification. I from (BIIHII)*-.A solution of ( { C ~ H ~ } ~ N ) ~ ( (0.3 B ~ Ig,H0.5 I~) mmol) and nickelocene (0.1 g, 0.5 mmol) in 1,2-dimethoxyethane was refluxed for 2 h to give a yellow solution. The product was deposited on alumina by removal of the solvent under reduced pressure and added to the top of a previously prepared 30 X 300 mm alumina chromatography column. Eluting with dichloromethane produced a yellow fraction which was identified as the tetrabutylammonium salt of 1 by IH and "B N M R spectroscopy. This fraction could be recrystallized with difficulty by addition of a small amount of ({C4H914N)Cl to a solution in acetone/EtOH/HzO and slowly removing the solvent under reduced pressure to yield 0.12 g (50%) of (1C4H9)4N)[(t15-C~HjNi)B~ I H I11). [(q5-C5H5Ni)BloH13](11) from Bl"H14. Nickelocene (1.9 g, 10 mmol) was added to a solution of decaborane (1.2 g, I O mmol) in 1,2-dimethoxyethane. An immediate reaction ensued with evolution of gas. ( I f the solvent is slightly wet, gentle heating may be required to initiate the reaction.) After 30 min the volume was reduced on a rotary evaporator and the solution filtered in the air, yielding a crude, brown product. The red-brown I I was obtained by recrystallization from benzene, I .5 g (60%). Further purification was achieved by chromatography on Florisil with hexane/acetone as eluent. The species 111 was obtained by treating an acetone solution of I1 with basic aqueous tetramethylammonium chloride. ((CH314N)1(115-C5H5Ni)Bl"Hlz] (111) from (B10H13)-. Treatment of 4.0 g (32 mmol) of decaborane with 1.7 g of N a H (50% in oil) in 150 mL of 1,2-dimethoxyethane resulted in the formation of NaBloH13. Addition of 7.0 g (37 mmol) of nickelocene and 20 g of 0.5% sodium amalgam resulted in gas evolution and change of the solution color from green to dark red. The solution was stirred for 45 min and the solvent was removed under reduced pressure. The residue was carefully treated with ice water and filtered through Celite when the foaming subsided. Addition of tetramethylammonium chloride produced a precipitate which was isolated by filtration. The solid was thoroughly extracted with acetone leaving behind the partially soluble ( I C H ~ } ~ N ) ~ [ N ~ ( B , OAddition H , , ) ~ ] .of ethanol to the acetone solution followed by slow removal of the solvent on a rotary evaporator produced 4.5 g (44% based on B10H14)of dark red crystals of 111 after drying under high vacuum. (lC~H~~~As)[(l~5-C5H512C~Ni)B~~H~~I (IV) from [(?-CsH5Ni)B10H121- (Ill).A solution of NaCcHS was prepared by the addition of 6 g of NaH (50% in oil) to 5 mL of freshly distilled cyclopentadiene in 100 mL of 1.2-dimethoxyethane. T o this were added 3 g of CoCl2 and 1 g of 111 and the solution was stirred for 12 h at room temperature. A stream of air was bubbled through the solution for 10 min and the solution filtered through Celite (caution: unreacted N a H may ignite if the filter cake is allowed to dry). The residue was washed once with acetonitrile and the mixture deposited on alumina by removal of the solvent under reduced pressure. The solids were placed on the top of a previously prepared alumina chromatography column. Eluting with dichloromethane gradually enriched with acetonitrile resulted in a red fraction containing a small amount of I1 and a bright green fraction. The green fraction was further purified by preparative thick layer chromatography on silica gel, then precipitated from an acetone/water solution with tetraphenylarsonium chloride. Recrystallization from ethanol/acetone and drying under high vacuum resulted in 0.25 g (10%) of bright green needles of IV. [ 1,2-(q5-C5H5Ni)zBloH10] (V). A. From Cyclopentadienylnickel Carbonyl Dimer. [(q5-CjH~NiC0)2] (304 mg, 1 mmol) and KzBloHlo (200 mg, 1 mmol) were stirred together in 50 m L of dry 1,2-dimethoxyethane at ambient temperature for 24 h. The solvent was then removed with a water aspirator. The dark brown, sticky residue was then slurried with 200 mL of 50:50 CH3CN/CH2C12 and 20 g of Florisil. This mixture w'as slowly spun dry, scraped from the wall of the round-bottom flask, and dried a t reduced pressure. The Florisil was then mounted on a 25 X 200 mm column of Florisil and eluted progressively with 50-60 "C, 40-45 OC, and boiling CHzCI2/hexane mixtures. A very fast band of unreacted dimer came down first, followed by a golden-brown band of the complex, V. Total yield was 86 mg (23%). Continued elution with CHzCl2 followed by CH2C12/ CH3CN mixtures yielded a dark brown tar containing no B-H (from infrared). B. From Nickelocene. Nickelocene (380 mg, 2 mmol) and KSBIOHIO

1 100:12 1 June 7 , 1978

(200 mg, 1 mmol) were brought to reflux in 50 mL dry 1,2-dimethoxyethane with magnetic stirring. The reaction mixture was maintained under reflux until the discharge of the green solution color (approximately 4 h). The workup was achieved as in method A. Total yield of V was 167 mg (45%). C. From [(CsH5)~Niz]+.Triscyclopentadienyldinickel tetrafluoroborate (200 mg, 0.25 mmol) and K2BloHlo(50 mg, 0.13 mmol) were stirred in 25 m L of dry 1,2-dimethoxyethane for 1 h. The reaction was extremely rapid. Isolation and workup were identical with those of method A. Total yield of V was 35 mg (73%). ((CH&N)[2-(vs-CsH~Ni)BgHg](VI) and ((CH&N)[1 -(q5-C5H5Ni)BsHs] (VII). A. From (B9Hl$. T o a solution of 1.8 g ( 1 0 mmol) of ({CH314N)(B9H12) and 1.8g (10mmol) of nickelocenein l 0 0 m L o f acetonitrile was added 50 g of 0.5% sodium amalgam. The solution was stirred for 1 h and then filtered through Celite. The mixture was deposited on alumina by removal of the solvent and placed at the top of a previously prepared 30 X 500 mm alumina chromatography column. Eluting with dichloromethane gradually enriched in acetonitrile first gave small amounts of several neutral compounds which were either unstable or did not contain boron, then the bright yellow fraction VI1 closely followed by the red complex VI. A green compound overlapped and followed VI. Eluting with CH3CN produced a small amount of an orange complex. Recrystallization of the red and yellow complexes was accomplished by adding a small amount of ((CH3J4N)CI to an acetone/HzO solution and slowly removing the acetone under reduced pressure a t ambient temperature. The crystalline products were isolated by filtration and dried under vacuum to yield 0.55 g (1 8%) of VI and 0.20 g (7%) of VII. The green fraction was evaporated under pressure and the residue (90% complete in 40 min. C. Photolytic. The photolytic isomerization of [2-(a5-CsHsNi)BgHgl- to [ 1-(q5-C5H~Ni)BgH9]-was performed by exposing the sample to a 50-W low-pressure mercury lamp. I n a typical experiment 20 mg of ((CH3}4N)[2-(15-CsHsNi)B9H~] was dissolved i n 2 mL of degassed CH3CN and sealed in a Pyrex N M R tube. The sample was then placed in an immersion well 2 in. from the lamp and the ' H N M R spectrum was periodically recorded. The isomerization was >90% complete in 20 min. Isomerization of [2-(CsHsNi)B9C19]- and [ l-(CsHsNi)BgC19]-. A. Thermal. The thermal isomerization was monitored using a thermostated UV-vis spectrometer by following the disappearance of the band at 664 nm. With the concentration of ((CH&N)[2-(C5H5Ni)B9C191 equal to 8.0 X M the isomerization was only 10% complete at 70 "C over 24 h. The isomerization was alternatively monitored with ' H N M R by integrating the (CsH5)- resonances of the equatorial ( T 4.36) and apical ( 7 4.40) isomers. The isomerization was >900? complete at 130 " C in 8 h. B. Photolytic. The photolyses of 1 - and [2-(+CsH5Ni)B&19]were carried out using a 500-W tungsten iodide lamp and monitoring the solution by ' H N M R . After 20 min the photolysis of either the apical or the equatorial isomer resulted in a 2: I apical/equatorial equilibrium mixture.

Acknowledgment.W e would like to thank Dr. C. G. Salentine for the IlB NMR. This work was supported in part by the Office of Naval Research. References and Notes (1) P. A. Wegner in "Boron Hydride Chemistry", E. L. Muetterties, Ed., Academic Press, New York, N.Y.. 1975, Chapter 12. (2) R. N. Grimes, "Carboranes", Academic Press, New York, N.Y., 1970. (3) (a) K. Wade, C k m . Commun., 792 (1971); (b) C. J. Jones, W. J. Evans, and M. F. Hawthorne, J. Chem. Soc., Chem. Commun., 543 (1973). (4) W. H. Knoth, H. C. Miller, 0.C. England, C. W. Parshall, and E. L. Muetterties, J. Am. Chem. Soc., 84, 1056 (1962). (5) 8.P. Sullivan, R. N. Leyden. and M. F. Hawthorne, J. Am. Chem. Soc.,97, 455 (1975). (6) R. N. Leyden and M. F. Hawthorne, J. Chem. Soc., Chem. Commun., 310 (1975).

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(7) M. F. Hawthorne and G. B. Dunks, Science, 176, 462 (1972). (8) D. F. Dustin, G. B. Dunks, and M. F. Hawthorne, J. Am. Chern. SOC.,95, 1109 (1973). (9) W. J. Evans and M. F. Hawthorne, J. Am. Chem. SOC.,93, 3063 (1971). (10) W. H. Knoth, J. Am. Chem. SOC.,89, 1274 (1967). (11) V. R. Miller and R. N. Grimes J. Am. Chem. Soc., 95, 5078 (1973). (12) V. R. Miller, R. Weiss. and R. N. Grimes J. Am. Chem. Soc., 99, 5646 (1977). (13) F. Klanberg, P. A. Wegner, G. W. Parshaii, and E. L. Muetterties, Inorg. Chem., 7, 2072 (1968). (14) A. R. Siedie and T. A. Hili, J. Inorg. Nucl. Chem., 31, 3874 (1969). (15) W. R. Pretzer and R. W. Rudolph, J. Am. Chem. SOC.,95, 932 (1973). (16) S. Pleiek and S. Heimanek, J. Chem. SOC., Chem. Commun., 127 (1975). 17) A possible explanation of this effect is that paramagnetic ring currents flow on the icosahedral surface oriented so that the circular paths travel around the molecular dipole axis. This results in a downfield shift for the boron above the plane of the rings, Le., the antipodal boron. Similar effects have been observed for proton chemical shifts in the annulenes. (SeeJ. A. Pople and K. G. Urtch. J. Am. Chern. SOC.,88, 484 (1966)). 18) G. Wilkinson, P. L. Paulson, and F. A. Cotton, J. Am. Chem. SOC.,76, 1970 (1954). 19) R. C. Dobbie, M. Green, and F. G. A. Stone, J. Chem. Soc. A, 1881 (1969). 20) M. E. Switzer and M. F. Renig, J. Chem. SOC., Chem. Commun., 687 (1972). (21) The Ni-Co icosahedral bimetalloborane anion is less thermally and hydrolytically stable than the Ni-Ni icosahedral bimetalioborane (vide infra). (22) H. Werner and A. Saker. Synth. Inorg. Met.-Org. Chem., 2, 239 (1972). (23) This is two mass units lower than the expected ( d e 365) peak. It is noted, however, that many metalloboranes lose H2 on pyrolysis to form metal borides. (24) The attempted rearrangement of this compound at 350 "C, in the solid state, yielded a dark black, nonmagnetic substance with no B-H, by infrared spectroscopy. It was slowly dissolved by dilute aqueous HCI to yield a green solution. (25) R. R. Rietz, D. F. Dustin, and M. F. Hawthorne, Inorg. Chem., 13, 1580 (1974). (26) C. G. Salentine, R. R. Rietz, and M. F. Hawthorne, Inorg. Chem., 13, 3025 (1974). (27) See relevant chapters in ref 1. (28) M. Green, J. Spencer, F. G. A. Stone, and A. Welch, J. Chem. Soc., Dalton Trans., 179 (1975). (29) F. G. A. Stone, J. Organomet. Chem., 100, 263 (1975). (30) V. R. Miller and R. N. Grimes, J. Am. Chem. Soc., 95,5078 (1973). (31) T. E. Paxson and M. F. Hawthorne, Inorg. Chem., 14, 1604 (1975). (32) The first observed photochemistryof metallocarboraneshas recently been reported: B. P. Sullivan, C. J. Jones, and M. F. Hawthorne, horg Chem., submitted. (33) M. F. Hawthorne and R. L. Piliing, J. Am. Chem. Soc., 88, 3873 (1966). (34) D. F. Dustin and M. F. Hawthorne, J. Am. Chem. Soc., 96, 3462 (1974). (35) L. F. Warren and M. F. Hawthorne, J. Am. Chem. Soc., 92, 1157 (1970). (36) J. G. Lawless, D. E. Bantak. and M. D. Hawley, J. Am. Chem. SOC.,91,7121 (1969). (37) C. J. Jones, J. N. Francis, and M. F. Hawthorne, J. Am. Chem. Soc., 95, 7633 (1973). (38) R. B. King, "Organometallic Syntheses", Vol. 1, Academic Press, New York, N.Y ., 1965, pp 70-7 I . (39) W. J. Jolly, Inorg. Synth., 11, 24 (1968). (40) S. Y. Tyree, Jr.. Inorg. Synth., 9, 16 (1967). (41) E. L. Muetterties, Inorg. Chem., 2, 647 (1963). (42) J. C. Carter and P. H. Wilks, Inorg. Chem., 9, 1777 (1970). (43) B. M. Graybill, J. K. Ruff, and M. F. Hawthorne, J. Am. Chem. Soc.,83,2669 (1961). (44) W. J. Jolly, "Synthesis and Characterization of inorganic Compounds", Prentice-Hall, Engiewood Cliffs, N.J., 1970. (45) D. V. Howe, C. J. Jones, R. J. Wiersema, and M. F. Hawthorne, Inorg. Chem., I O , 2516 (1971).