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Simple bonding schemes for eight-vertex D2d dodecahedral clusters violating Wade's rules
132
NOTES
(a)
(b)
Y
Fig. 1. 9sMo NMR spectra of ChzCls solutions of (a) Mo(CO)rPPhs (1293 transients), (b) cis-Mo(CO),(MePPhs)z (944 transients), (c) truns-Mo(CO),(MePPhz)s(1791transients), and (d) Mo(CO)s(MePPhr)s(894 transients). Scales are different, but the bar represents 100Hz in each case.
Polyhedron Vol. I. No. 1. PP. 13343.4. Printed in Great Britain.
1982
0277-5387/82/010133-o2so3.00/0 Per~mon PressLtd.
Simple bonding schemes for eight-vertex Dzr dodeeahedral clusters violating Wade’s rules (Receioed 16 September 1981) The geometry of deltahedral eight-vertex clusters such as borane and carborane derivatives is based on the Dr.+dodecahedron, the topology of which can be represented as a cube with six added diagonals (l).’ Most such clusters (e.g. BsHar- and C2B6Hs) contain the 18 (namely 2n +2) skeletal electrons’ expected from Wade’s rules315 and consistent with delocalized bonding
models6 However, recently both an apparent 16 skeletal electron tetracobalt complex (C,H,),Co,B& (Ref. 2) and an apparent 20 skeletal electron tetranickel complex (C5HJ),N&B,H, (Refs. 7 and 8) have been shown also to exhibit Dad dodecahedral geometry thereby violating Wade’s rules. Recent extended Huckel molecular orbital calculations9 on these systems interpreted to
NOTES
133
I
contain two internenetratina tetrahedra orovide a basis for these violations of Wade’s rules. This correspondence shows how much simpler bonding models are sufficient to account for these discrepancies. Consider first the nickel complex (CsHs),N&B,Hdshown by an X-ray structural determination7a to have the CsHsNi units at the vertices of degree 4 (namely 2,3,6 and 7 in I) and the BH units at the vertices of degree 5 (namely I, 4,5, and 8 in 1). The Ni-Ni distances along the diagonals 23 and 67 (I) are 2.35 A consistent with a Ni-Ni bond. The remaining Ni-Ni distances fall in the range 3.563.6OA which are-too long for Ni-Ni bonding. Both the nickel and boron atoms have the favoured electronic configuration of the next rare gas if there are localized covalent Ni-Ni bonds along the 23 and 67 diagonals and localized NCB bonds along each of the twelve original edges of the cube. Thus each boron atom is bonded to three nickel atoms and each nickel atom is bonded to three boron atoms. The environment around each boron atom can be represented as II (M = Ni) in which one of the three metal atoms forms a dative metal boron bond and the other two metal atoms form normal metalboron covalent bonds. Through resonance all three of these metal-boron two-center bonds can become equivalent. Each nickel atom acquires the l8-electron rare gas configuration as follows: (a) IO electrons from neutral nickel; (b) 5 electrons from neutral CsHs; (c) 1 electron from the Ni-Ni bond across diagonal 23 or 67; (d) I electron each (total 2 electrons) from the two BH groups with which the nickel atom forms a normal two-centre covalent bond; (e) no electrons from the third BH group, namely B dative the one with which the nickel atom forms the Nibond. This bonding model for (CsHs),N&B,H, consists of fourteen two-centre bonds and no three-centre bonds. The two-centre bonds are found on all of the eighteen edges of the Dzddodecahedron (1) except for the 18, 84,45 and 51 edges. This bonding model has the following difficulties: (I) The 18, 84, 45 and 51 “non-bonding” edges in this model correspond to B-B distances which are found by X-ray crystallography’s to be close enough (1.87-I .% A) to imply B-B bonding; (2) The C,HcNi vertices use four internal orbitals rather than the three internal orbitals normally used by CsHsNi vertices in deltahedra6 such as (C,H,),Ni,B,,,H,,, _ _._ _ _ _ and related molecules.8 This makes the CsHsNi vertices in (CsHs),Ni,B,H, effective five skeletal electron donors rather than the more normal type of three skeletal electron donor found for a CsHsNi vertex using three internal orbitals. By such electron counting rules (CsHs),Ni,B,H, would be a (4)(5) t (4)(2) = 28 skeletal electron system consistent with the fourteen two-centre bonds implied by this model. These difficulties can be circumvented by applying operation III (M = Ni) four times. Each application of this operation trades two two-centre bonds for one three-centre bond and removes one electron pair of the CsHsNi vertex from the skeletal electron system. We thus arrive at a (CsHs),NLB,H, polyhedron with 4 three-centre bonds and I4 - (2)(4)= 6 two-centre bonds requiring a total of twenty skeletal electrons for the ten bonds. This is exactly what is available since operation III restores a CsHsNi vertex to the usual donor of three skeletal electrons and three internal orbitals.
An analogous situation applies to the cobalt complex (CsHs),Co,B&. However, its structure determination by X-ray diffraction shows a completely different arrangement of metal and boron vertices in the Ds4 dodecahedron. Thus the CsHsCo units now appear at the vertices of degree 5 (namely I, 4,5, and 8 in I) and the BH units at the vertices of degree 4 (namely 2,3,6 and 7 in 1). There are now four bonding Co-Co distances (- 2.48A), namely those along the four diagonals IS, 54. 48, and 81. The remaining two Co-Co distances (- 3.18A) are much longer indicating no direct Co-Co bonding. Each cobalt atom is therefore directly bonded to two other cobalt atoms in ordinary two-electron localized bonds. The Co, unit may be considered as a quadrilateral which is puckered so that each of the four boron atoms can bond to a different set of three cobalt atoms. Each boron atom thus has the same rare gas electronic configuration (II) in both (CsHs)&B,H,(M=Co and Ni) complexes. Each cobalt atom in (CsHs),Co,BJI, acquires the I8 electron rare gas configuration as follows: (a) 9 electrons from the neutral cobalt; (b) 5 electrons from the neutral CsHs; (c) I electron each (total 2 electrons) from the two cobalt atoms to which it is directly bonded; (d) I electron each (total 2 electrons) from the two BH groups with which the cobalt forms a normal covalent bond; (e) no electrons from the third BH group, namely the one with which the cobalt forms a Co---, B dative bond. This bonding model for (CsHs)&o,B,H, consists of sixteen two-centre bonds and no threecentre bonds. The two-centre bonds are found on all of the eighteen edges of the Dsd dodecahedron (I) except for the 23 and 67 edges. This bonding model has difficulties analogous to those discussed above for the bonding model of (CsHs),Ni,B,H, containing only two-centre bonds. These difficulties can be circumvented by applying operation III (M = Co) four times and operation IV two times. Operation IV, like operation III converts 2T two-centre bonds of specified type into a group of T three-centre bonds (T = I for operation III and T = 2 for operation IV). We thus arrive at a (CsH&Co,B,H, polyhedron with eight three-centre bonds and no two-centre bonds. The eight three-centre bonds, of course, are fully consistent with the observed sixteen skeletal electrons of (CsHs),Co,B,H, calculated on the basis of each CsHsCo vertex being a donor of two skeletal electrons and three internal orbitals in the conventional way. Previous work” indicates that the relative energetics of alternative structures in eight-vertex deltahedral boranes are more delicate than those in deltahedral boranes of other sizes. The analysis in this paper suggests a new delicacy of the eight vertex deltahedral system, namely the replacement of light atom vertices (boron and carbon) by transition metal vertices leads eventually to a point where deltahedral bonding involving both surface and core interactions (namely a 2n t 2 skeletal electron system using three internal orbital8 of each vertex) is replaced by surface localized bonding. Furthermore, this paper shows how such surface localized bonding can be described either by using only two-center bonds and variable numbers of internal orbitals from different vertex atoms or by using three-centre bonds (and possibly some two-centre bonds as well) and three internal orbitals from each vertex atom. Such alternative bonding models can be related by applications of the relatively simple operations III and IV. Acknowlnlgrments-I am indebted to the U.S. Army Research Office for partial support of this work under Contract No.
NOTES
134
DAAG29-80-KOO30, I am also indebted to Dr. Stanislav Heimanek of the Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, Dr. D. hf. P. Mingos of the Inorganic Chemistry Laboratory, Oxford University (England), and Dr. Russell Grimes of the University of Virginia for useful suggestions and constructive criticisms of earlier versions of this paper. Departmentof Chemistry Universityof Georgia Athens, GA 30602 U.S.A.
R. B. KING
REFERENCES ‘R. B. King, Theor. Chim. Acta., 1981,59,25.
Polyhedron Vol. I. No. 1. pp. 134435, Printed in Great Britain.
‘J. R. Pipal and R. N. Grimes, Inorg. Chem., 1979,18,257, ‘K. Wade, Chem. Comm.;‘1971,792. ‘R. N. Grimes, Ann. N. Y. Acad. Sci., 1974,239, 180. ‘K. Wade, Ado. Inorn. Chem. Radiochem., 1976.18. 1. 6R.B. King and D. H.Rouvray, J. Am. Chem. Six, 1977,99,7834. ‘5. R. Bowser and R. N. Grimes, 1. Am. Chem. Sot., 1978,100, 4623. ‘5. R. Bowser, A. Bonny, J. R. Pipal and R. N. Grimes, J. Am. Chem. SIC.. 1979.101.6229. 9D.N. Cox, fi. M. 6. Mi&os and R. Hoffmann, J.C.S. Dalton, 1981, 1788. “‘E.L. Muetterties and B. F. Beier, Bull. Sot. Chim.Belg., 1978.84, 397.
1982
can-5387/82/010l344$03.@3/0 PergamonPressLtd.
New routes to bslogenated Bs and Be horon cages (Received 17thJu/y 1981) AhstraceBsBrs and B9Br9are formed when B&is and B&l9 are heated with aluminium tribromide. Cage-size reduction occurs on heating B&l,,, and B&l,, with hydrogen to give $CisH and B9CI,Hz,respectively. At least six bromine atoms in B9Br9can be. substituted for methyl groups using SnMe,.
To date complete halogen exchange on boron cage compounds has not been achieved. However, we have found that B&Is can be fully brominated under the relatively mild conditions of 100°C in the presence of aluminium tribromide using boron tribromide as solvent. B&l9 does not react under these conditions but is brominated completely by molten aluminium tribromide (which acts as a solvent) at 260°C. Normally, B&h as isolated from decomposed diboron tetrachloride samples contains substantial amounts of B&l, which are very difficult to remove;’ the differing reactivities of the Bs and B9 systems towards brominechlorine exchange means that separation of the two chlorides is not required prior to reaction. The B&XB&l9 mixture was sealed under vacuum in a Pyrex tube with freshly sublimed aluminium tribromide and vacuum distilled boron tribromide to give a very dark purple solution (the colour being due to BsCls). When the tube was heated to loo” the colour was observed to slowly change to dark brown; after 14 days the tube was opened under vacuum and the boron tribromide removed. The gentle heat of a hot air blower was sufficient to sublime the ahuninium halides and unreacted B&l9 to a remote part of the apparatus leaving behind a dark red-brown solid. This solid sublimed cleanly on heating with a free flame and was identified as BsBrs by mass spectrometry. On resealing the tube containing the yellow mixture of aluminium halides and B&l9 and heating to 260°C for 15hr it was found that the colour slowly changed to deep red; fractional sublimation of the products allowed isolation of pure B9Br9as dark red crystals. To our knowledge BsBrs has not been isolated previously although it has been detected as a minor component among the decomposition products of diboron tetrabromide? It is a very dark reddish-brown, water-
sensitive solid which is soluble in halogenated solvents and which sublimes without melting when heated under vacuum. The parent ion gives rise to the base peak of the mass spectrum, the next most intense peak being due to the loss of BBrs from the parent ion. An as yet unexplained phenomenon is that glass having been in contact with B,Br8 and then sealed with an oxygen-gas flame assumes a light green colour near the point of sealing; in contrast, glass previously used for handling B&l8 takes on a permanganate-purple colour at the seal. The other boron sub-halides do not exhibit this glasscolouring effect. Previously we have described the reaction of B,&l,, B,,Cl,, mixtures with hydrogen? To study the reaction of BloCllo alone with hydrogen the BIOCIIO-BIICIII mixtures obtained from decomposed diboron tetrachloride were heated under vacuum at 350” to pyrolyse the less stable B,,Cl,, leaving the B,&llo intact (small amounts of B&l9 sometimes formed in this procedure do not react with hydrogen below 3OO’C”and so do not affect the next stage of reaction). The red-orange BloCllo was sublimed into a clean piece of apparatus, sealed up with lO-20cm pressure of hydrogen and heated to 150”overnight. On cooling a yellow, crystalline solid condensed out on the cooler parts of the glass tubing; mass spectral analysis showed this to be B&&H contaminated with unchanged B&19. The main fragmentation process in the mass spectrometer is loss of either BC13or BCl*H from the parent ion to give B&b+ and B&H’; the next two most prominent ions were B&13+ and B,Cl,‘. If, as seems likely, the BloCllo molecule possesses a bicapped square antiprismatic boron cage a possible mechanism for the exclusive formation of B&&H may be as follows. Loss of one of the eight equatorial boron atoms from the cage (boron 2) would leave the pre-
NOTES
(a)
(b)
Y
Fig. 1. 9sMo NMR spectra of ChzCls solutions of (a) Mo(CO)rPPhs (1293 transients), (b) cis-Mo(CO),(MePPhs)z (944 transients), (c) truns-Mo(CO),(MePPhz)s(1791transients), and (d) Mo(CO)s(MePPhr)s(894 transients). Scales are different, but the bar represents 100Hz in each case.
Polyhedron Vol. I. No. 1. PP. 13343.4. Printed in Great Britain.
1982
0277-5387/82/010133-o2so3.00/0 Per~mon PressLtd.
Simple bonding schemes for eight-vertex Dzr dodeeahedral clusters violating Wade’s rules (Receioed 16 September 1981) The geometry of deltahedral eight-vertex clusters such as borane and carborane derivatives is based on the Dr.+dodecahedron, the topology of which can be represented as a cube with six added diagonals (l).’ Most such clusters (e.g. BsHar- and C2B6Hs) contain the 18 (namely 2n +2) skeletal electrons’ expected from Wade’s rules315 and consistent with delocalized bonding
models6 However, recently both an apparent 16 skeletal electron tetracobalt complex (C,H,),Co,B& (Ref. 2) and an apparent 20 skeletal electron tetranickel complex (C5HJ),N&B,H, (Refs. 7 and 8) have been shown also to exhibit Dad dodecahedral geometry thereby violating Wade’s rules. Recent extended Huckel molecular orbital calculations9 on these systems interpreted to
NOTES
133
I
contain two internenetratina tetrahedra orovide a basis for these violations of Wade’s rules. This correspondence shows how much simpler bonding models are sufficient to account for these discrepancies. Consider first the nickel complex (CsHs),N&B,Hdshown by an X-ray structural determination7a to have the CsHsNi units at the vertices of degree 4 (namely 2,3,6 and 7 in I) and the BH units at the vertices of degree 5 (namely I, 4,5, and 8 in 1). The Ni-Ni distances along the diagonals 23 and 67 (I) are 2.35 A consistent with a Ni-Ni bond. The remaining Ni-Ni distances fall in the range 3.563.6OA which are-too long for Ni-Ni bonding. Both the nickel and boron atoms have the favoured electronic configuration of the next rare gas if there are localized covalent Ni-Ni bonds along the 23 and 67 diagonals and localized NCB bonds along each of the twelve original edges of the cube. Thus each boron atom is bonded to three nickel atoms and each nickel atom is bonded to three boron atoms. The environment around each boron atom can be represented as II (M = Ni) in which one of the three metal atoms forms a dative metal boron bond and the other two metal atoms form normal metalboron covalent bonds. Through resonance all three of these metal-boron two-center bonds can become equivalent. Each nickel atom acquires the l8-electron rare gas configuration as follows: (a) IO electrons from neutral nickel; (b) 5 electrons from neutral CsHs; (c) 1 electron from the Ni-Ni bond across diagonal 23 or 67; (d) I electron each (total 2 electrons) from the two BH groups with which the nickel atom forms a normal two-centre covalent bond; (e) no electrons from the third BH group, namely B dative the one with which the nickel atom forms the Nibond. This bonding model for (CsHs),N&B,H, consists of fourteen two-centre bonds and no three-centre bonds. The two-centre bonds are found on all of the eighteen edges of the Dzddodecahedron (1) except for the 18, 84,45 and 51 edges. This bonding model has the following difficulties: (I) The 18, 84, 45 and 51 “non-bonding” edges in this model correspond to B-B distances which are found by X-ray crystallography’s to be close enough (1.87-I .% A) to imply B-B bonding; (2) The C,HcNi vertices use four internal orbitals rather than the three internal orbitals normally used by CsHsNi vertices in deltahedra6 such as (C,H,),Ni,B,,,H,,, _ _._ _ _ _ and related molecules.8 This makes the CsHsNi vertices in (CsHs),Ni,B,H, effective five skeletal electron donors rather than the more normal type of three skeletal electron donor found for a CsHsNi vertex using three internal orbitals. By such electron counting rules (CsHs),Ni,B,H, would be a (4)(5) t (4)(2) = 28 skeletal electron system consistent with the fourteen two-centre bonds implied by this model. These difficulties can be circumvented by applying operation III (M = Ni) four times. Each application of this operation trades two two-centre bonds for one three-centre bond and removes one electron pair of the CsHsNi vertex from the skeletal electron system. We thus arrive at a (CsHs),NLB,H, polyhedron with 4 three-centre bonds and I4 - (2)(4)= 6 two-centre bonds requiring a total of twenty skeletal electrons for the ten bonds. This is exactly what is available since operation III restores a CsHsNi vertex to the usual donor of three skeletal electrons and three internal orbitals.
An analogous situation applies to the cobalt complex (CsHs),Co,B&. However, its structure determination by X-ray diffraction shows a completely different arrangement of metal and boron vertices in the Ds4 dodecahedron. Thus the CsHsCo units now appear at the vertices of degree 5 (namely I, 4,5, and 8 in I) and the BH units at the vertices of degree 4 (namely 2,3,6 and 7 in 1). There are now four bonding Co-Co distances (- 2.48A), namely those along the four diagonals IS, 54. 48, and 81. The remaining two Co-Co distances (- 3.18A) are much longer indicating no direct Co-Co bonding. Each cobalt atom is therefore directly bonded to two other cobalt atoms in ordinary two-electron localized bonds. The Co, unit may be considered as a quadrilateral which is puckered so that each of the four boron atoms can bond to a different set of three cobalt atoms. Each boron atom thus has the same rare gas electronic configuration (II) in both (CsHs)&B,H,(M=Co and Ni) complexes. Each cobalt atom in (CsHs),Co,BJI, acquires the I8 electron rare gas configuration as follows: (a) 9 electrons from the neutral cobalt; (b) 5 electrons from the neutral CsHs; (c) I electron each (total 2 electrons) from the two cobalt atoms to which it is directly bonded; (d) I electron each (total 2 electrons) from the two BH groups with which the cobalt forms a normal covalent bond; (e) no electrons from the third BH group, namely the one with which the cobalt forms a Co---, B dative bond. This bonding model for (CsHs)&o,B,H, consists of sixteen two-centre bonds and no threecentre bonds. The two-centre bonds are found on all of the eighteen edges of the Dsd dodecahedron (I) except for the 23 and 67 edges. This bonding model has difficulties analogous to those discussed above for the bonding model of (CsHs),Ni,B,H, containing only two-centre bonds. These difficulties can be circumvented by applying operation III (M = Co) four times and operation IV two times. Operation IV, like operation III converts 2T two-centre bonds of specified type into a group of T three-centre bonds (T = I for operation III and T = 2 for operation IV). We thus arrive at a (CsH&Co,B,H, polyhedron with eight three-centre bonds and no two-centre bonds. The eight three-centre bonds, of course, are fully consistent with the observed sixteen skeletal electrons of (CsHs),Co,B,H, calculated on the basis of each CsHsCo vertex being a donor of two skeletal electrons and three internal orbitals in the conventional way. Previous work” indicates that the relative energetics of alternative structures in eight-vertex deltahedral boranes are more delicate than those in deltahedral boranes of other sizes. The analysis in this paper suggests a new delicacy of the eight vertex deltahedral system, namely the replacement of light atom vertices (boron and carbon) by transition metal vertices leads eventually to a point where deltahedral bonding involving both surface and core interactions (namely a 2n t 2 skeletal electron system using three internal orbital8 of each vertex) is replaced by surface localized bonding. Furthermore, this paper shows how such surface localized bonding can be described either by using only two-center bonds and variable numbers of internal orbitals from different vertex atoms or by using three-centre bonds (and possibly some two-centre bonds as well) and three internal orbitals from each vertex atom. Such alternative bonding models can be related by applications of the relatively simple operations III and IV. Acknowlnlgrments-I am indebted to the U.S. Army Research Office for partial support of this work under Contract No.
NOTES
134
DAAG29-80-KOO30, I am also indebted to Dr. Stanislav Heimanek of the Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, Dr. D. hf. P. Mingos of the Inorganic Chemistry Laboratory, Oxford University (England), and Dr. Russell Grimes of the University of Virginia for useful suggestions and constructive criticisms of earlier versions of this paper. Departmentof Chemistry Universityof Georgia Athens, GA 30602 U.S.A.
R. B. KING
REFERENCES ‘R. B. King, Theor. Chim. Acta., 1981,59,25.
Polyhedron Vol. I. No. 1. pp. 134435, Printed in Great Britain.
‘J. R. Pipal and R. N. Grimes, Inorg. Chem., 1979,18,257, ‘K. Wade, Chem. Comm.;‘1971,792. ‘R. N. Grimes, Ann. N. Y. Acad. Sci., 1974,239, 180. ‘K. Wade, Ado. Inorn. Chem. Radiochem., 1976.18. 1. 6R.B. King and D. H.Rouvray, J. Am. Chem. Six, 1977,99,7834. ‘5. R. Bowser and R. N. Grimes, 1. Am. Chem. Sot., 1978,100, 4623. ‘5. R. Bowser, A. Bonny, J. R. Pipal and R. N. Grimes, J. Am. Chem. SIC.. 1979.101.6229. 9D.N. Cox, fi. M. 6. Mi&os and R. Hoffmann, J.C.S. Dalton, 1981, 1788. “‘E.L. Muetterties and B. F. Beier, Bull. Sot. Chim.Belg., 1978.84, 397.
1982
can-5387/82/010l344$03.@3/0 PergamonPressLtd.
New routes to bslogenated Bs and Be horon cages (Received 17thJu/y 1981) AhstraceBsBrs and B9Br9are formed when B&is and B&l9 are heated with aluminium tribromide. Cage-size reduction occurs on heating B&l,,, and B&l,, with hydrogen to give $CisH and B9CI,Hz,respectively. At least six bromine atoms in B9Br9can be. substituted for methyl groups using SnMe,.
To date complete halogen exchange on boron cage compounds has not been achieved. However, we have found that B&Is can be fully brominated under the relatively mild conditions of 100°C in the presence of aluminium tribromide using boron tribromide as solvent. B&l9 does not react under these conditions but is brominated completely by molten aluminium tribromide (which acts as a solvent) at 260°C. Normally, B&h as isolated from decomposed diboron tetrachloride samples contains substantial amounts of B&l, which are very difficult to remove;’ the differing reactivities of the Bs and B9 systems towards brominechlorine exchange means that separation of the two chlorides is not required prior to reaction. The B&XB&l9 mixture was sealed under vacuum in a Pyrex tube with freshly sublimed aluminium tribromide and vacuum distilled boron tribromide to give a very dark purple solution (the colour being due to BsCls). When the tube was heated to loo” the colour was observed to slowly change to dark brown; after 14 days the tube was opened under vacuum and the boron tribromide removed. The gentle heat of a hot air blower was sufficient to sublime the ahuninium halides and unreacted B&l9 to a remote part of the apparatus leaving behind a dark red-brown solid. This solid sublimed cleanly on heating with a free flame and was identified as BsBrs by mass spectrometry. On resealing the tube containing the yellow mixture of aluminium halides and B&l9 and heating to 260°C for 15hr it was found that the colour slowly changed to deep red; fractional sublimation of the products allowed isolation of pure B9Br9as dark red crystals. To our knowledge BsBrs has not been isolated previously although it has been detected as a minor component among the decomposition products of diboron tetrabromide? It is a very dark reddish-brown, water-
sensitive solid which is soluble in halogenated solvents and which sublimes without melting when heated under vacuum. The parent ion gives rise to the base peak of the mass spectrum, the next most intense peak being due to the loss of BBrs from the parent ion. An as yet unexplained phenomenon is that glass having been in contact with B,Br8 and then sealed with an oxygen-gas flame assumes a light green colour near the point of sealing; in contrast, glass previously used for handling B&l8 takes on a permanganate-purple colour at the seal. The other boron sub-halides do not exhibit this glasscolouring effect. Previously we have described the reaction of B,&l,, B,,Cl,, mixtures with hydrogen? To study the reaction of BloCllo alone with hydrogen the BIOCIIO-BIICIII mixtures obtained from decomposed diboron tetrachloride were heated under vacuum at 350” to pyrolyse the less stable B,,Cl,, leaving the B,&llo intact (small amounts of B&l9 sometimes formed in this procedure do not react with hydrogen below 3OO’C”and so do not affect the next stage of reaction). The red-orange BloCllo was sublimed into a clean piece of apparatus, sealed up with lO-20cm pressure of hydrogen and heated to 150”overnight. On cooling a yellow, crystalline solid condensed out on the cooler parts of the glass tubing; mass spectral analysis showed this to be B&&H contaminated with unchanged B&19. The main fragmentation process in the mass spectrometer is loss of either BC13or BCl*H from the parent ion to give B&b+ and B&H’; the next two most prominent ions were B&13+ and B,Cl,‘. If, as seems likely, the BloCllo molecule possesses a bicapped square antiprismatic boron cage a possible mechanism for the exclusive formation of B&&H may be as follows. Loss of one of the eight equatorial boron atoms from the cage (boron 2) would leave the pre-