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Adducts of trimethylaluminium with phosphine ligands: X-ray crystal structures of Me3AlPPh3 and Me3AlP(o-tolyl)3
Polyhedron Vol. 8, No. 6, pp. Printed in Great Britain
831-834,
1989 0
0277-5387/X9 %3.00+.00 1989 Pergamon Press
plc
ADDUCTS OF TRIMETHYLALUMINIUM WITH PHOSPHINE LIGANDS: X-RAY CRYSTAL STRUCTURES OF Me3AlPPh3 AND Me,AlP(o-tolyl), DERK A. WIERDA and ANDREW R. BARRON* Department of Chemistry, Harvard University, Cambridge, MA 02138, U.S.A. (Received 3 1 August 1988 ; accepted 24 October 1988) Abstract-The
crystal and molecular structures of two phosphine adducts of trimethylaluminium are reported. Compound 1, Me3AlPPh3, crystallizes in the rhombohedral space group R,(obv), a = b = 14.344(4), c = 16.503(7) A, I’= 2944(2) A3, 2 = 6, R = 0.0436, R, = 0.0532. Compound 2, Me,AlP(o-tolyl),, crystallizes in the rhombohedral space group R3C, a = b = 14.978(l), c = 34.195(5) A, V = 6643(l) A3, 2 = 4, R = 0.0574, R, = 0.0601. Structural features are discussed with reference to the 13C and 31P NMR spectroscopic data.
We have recently shown’ that the dominant factor influencing the NMR chemical shifts in Me3AlPR3 complexes is steric repulsion between the aluminium methyl groups and the phosphine substituents. The 13C chemical shifts of the A1-CH3 groups may be used as an indication of the distortion from an idealized planar, monomeric AlMe,, and the change in 3‘P chemical shift (A)? of the phosphines on coordination to AlMe, correlates with the difference in R-P-R angles between free and coordinated phosphine. The complex, Me3A1P(o-tolyl),, does not fit the trends found in 3‘P A or Al-CH, r3C chemical shifts. Based on a calculated cone angle for P(otolyl)32 of 194” a negative 3’P A is expected if the angle R-P-R decreases on coordination. The observed value, however, is positive (+ 7.7 ppm)’ indicating that the C-P-C angle is opened on coordination. This observation suggests that the P(o-tolyl), ligand adopts a conformation so as to relieve steric strain between the o-tolyl and aluminium methyl groups. In order to test the validity of the NMR data as a structural probe for Me3A1PR3 complexes we have undertaken an X-ray crystallographic study of Me,AlPPh, and Me3A1P(o-tolyl),.
RESULTS
AND DISCUSSION
Me3A1PPh3 The molecular structure of Me3AlPPh3 (1) is shown in Fig. 1 and selected bond lengths are given in Table 1. The complex contains a crystallographically-imposed C3 axis coincident with the Al-P bond. The aromatic rings of the triphenylphosphine ligand adopt a propeller-like conformation, with the phenyl groups pitched 38.5” from the plane defined by Al, P and C(1). As a consequence of this the Al-C and P-C bonds are only partly staggered with a dihedral angle of 39.8”.
*Author to whom correspondence should be addressed. t 3‘P A = &comp~cx - &re+osp,,ine.
Fig. 1. X-ray crystal structure of Me,AIPPh, (1) showing the numbering scheme.
831
D. A. WIERDA
832
and A. R. BARRON
Table 1. Selected bond lengths (A) and angles (“) for 1 2.535( 1) 1.820(2) 1.385(3) 1.395(3)
Al-P P--c(l) c(2F(3) C(5)--c(6) Al-P-C( 1) P--c(lHX2) c(ltc(2)--c(3) C(3)-C(4)----c(5) C(5)-C(6)----c(l) The shortest phenyl ring
Al-C( 10) C(lW(2) C(3)--c(4) C(6HYl)
114.0(l) 122.7( 1) 120.1(2) 119.8(2) 120.1(2)
non-bonding distances between and alnmininm methyl group
1.981(2) 1.391(3) 1.377(3) 1.394(2)
P-AI-C( 10) P---U)-C(6) C(2tc(3)-c(4) C(4)_-c(5)--c(6)
the are
C(2)--C(lOb) = 3.819(2) 8, and C(2)-C(lOA) = 3.993(2) A. These contacts are emphasized by viewing the molecule along Al-P (Fig. 2). The Al-P distance, 2.535(l) A, does not vary significantly from that found in Me,AlPMe3 (2.53(4) A)3 or (Me3A1),*(Ph,PCHJ2 (2.544(4) A).” A comparison of the structure of 1 and free monomeric A1Me35 (see Table 2) shows that the lengths of the AI-C bonds increase, and that the
100.6( 1) 119.0(2) 120.8(2) 120.2(2)
C-Al-C angles decrease on coordination. Both changes are in the direction predicted by an increased aluminium p-character in the Al-C bonds on changing from a planar to tetrahedral geometry. From NMR results’ the aluminium centre should be distorted further from planarity in Me,AlPPh3 than in Me3A1PMe3, this is indeed the case. The changes in PPh3 geometry6 on coordination are not so marked (see Table 2). The only appreciable change is an opening of the C-P-C angle by ca 1.6”. This is in contrast to the results from 31P NMR spectroscopy’ which predict that the C-P-C angle is reduced slightly on coordination. It is likely that this discrepancy between solid state and solution is a consequence of the distortions observed in the crystal structure of PPh 3.6 The unit cell of Me3AlPPh3 is comprised of six molecules with no unduly close intermolecular contacts.
Me,AlP(o-tolyl),
Fig. 2. The structure of Me,AlPPh3 (1)viewed down the Al-P vector.
The molecular structure of Me,A1P(o-tolyl)3 (2) is shown in Fig. 3 ; selected bond lengths and angles are given in Table 3. The complex contains a crys-
Table 2. Selected bond lengths (A) and angles (“) of AlMe,, Me,AIPRj phosphines, PR, Al-P
Compound Me,Al” Me,AIPMe,b PMe3’ Me,AlPPh, PPh, Me,AlP(o-tolyl), P(o-tolyl),d a Reference b Reference ’ Reference ‘Reference
5. 3. 6. 7.
Al-C
2.53
1.957 1.973
2.535
1.981
2.584
1.874
P-C
1.822 1.846 1.820 1.828 (av) 1.834 1.86 (av)
complexes and free
C-Al-C 120.0 117.1 116.6 113.9
C-P-C
103.4 97.54 104.6 102.9 (av) 105.1 102.8 (av)
Adducts of trimethylaluminium
with phosphine ligands
833
Table 3. Selected bond lengths (A) and angles (“) for 2 Al-C( 10) P-C(l) C(l>--c(6) C(3)--C(4) CW-C(6) C( lo)-Al-P P--c(l)-C(2) C(2)-C(l)--c(6) C(2)-C(3)--c(4) C(4)-C(5)--c(6) C(l)--c(6)-C(7)
1.974(6) 1.834(4) 1.400(5) 1.376(6) 1.377(7)
Al-P C(l)-C(2) C(2)-~(3) C(4)-W C(6)-C(7)
104.5(l) 119.3(2) 119.7(4) 120.1(5) 122.6(4) 123.1(4)
Fig. 3. X-ray crystal structure of Me,AlP(o-tolyl), showing the numbering scheme.
2.584(2) 1.402(6) 1.384(7) 1.381(9) 1.512(7)
Al-P-C( 1) P--c(l)-C(6) C(~4x2+C(3) C(3)--c(4)-C(5) C(l)--c(6)-C(5) C(5>--c(6)--c(7)
(2)
C3 axis along the Al-P bond. The aromatic rings of the o-tolyl groups adopt a propeller-like arrangement, with a pitch of 50.2” from the plane defined by Al, P and C(1). As a consequence of this, the Al-C and P-C bonds are nearly eclipsed, with a dihedral angle of 10.5”. This conformation allows the o-tolyl methyl groups to be staggered with respect to the altinium methyl groups (Fig. 4). The shortest non-bonding distances between the o-tolyl methyl and the aluminium methyl groups are C(7)--C( 10) = 3.48 l(4) A tallographically-imposed
113.6(l) 120.9(3) 120.3(3) 119.2(5) 118.1(4) 118.7(3)
and C(7)-C(10) = 3.872(5) A. The Al-P distance, 2..584(2) A, is significantly longer than Al-P distances previously reported. 3,4 The changes in the Al-C bond distance and C-Al-C angle on coordination of AlMe, to P(otolyl), (see Table 2) are consistent with increased aluminium p-character in the Al-C bonds on changing from a planar to tetrahedral geometry. From 13C NMR data it was predicted’ that the C-Al-C angle for 2 should be smaller than that found for 1; this is observed (Table 2). The 31P A value for 2 ’ suggested a larger C-P-C angle on coordination than in free phosphine,7 which is confirmed by the crystal structure (see Table 2). It is of course possible that the conformation of the o-tolyl rings observed in the crystal structure is not retained in solution. It is clear, however, that changes in the geometry of 2 predicted by NMR are consistent with those observed in the solid state. The unit cell of Me,AlP(o-tolyl), is comprised of four molecules with no unduly close intermolecular contacts. All the intermolecular distances correspond to, or are greater than, normal van der Waals contacts.
CONCLUSIONS The X-ray structural data reported here confirm that ’3C and 31P NMR data can be used as an indication of changes in geometry around aluminium and phosphorus on coordination of phosphines to AlMe,. There is no evidence, however, that NMR can be used to predict either the Al-P bond length, or the energy of dissociation of the complexes Me3AlPR3. EXPERIMENTAL Fig. 4. The structure of Me,AlP(o-tolyl), the Al-P vector.
(2) viewed down
Crystals were prepared as described in ref. 1 and sealed under nitrogen in Lindemann capillaries.
834
D. A. WIERDA
and A. R. BARRON
Table 4. Crystal data, details of intensity measurement and structure refinement for Me,AlPPh, (1) and Me,AlP(o-tolyl), (2) Compound Formula M, Crystal dimensions (mm) Crystal system a, b (A) c (A) v (A’) Space group
0, (g cm-‘) Z 1 (cm- ‘)
Minimum, maximum (“) 7’ (IQ Total data Total unique data Total observed data Significance test Number of parameters Weighting scheme Parameter g in l/[(F,)*+gF~] Final R Final R,
Unit-cell parameters and intensity data were detailed obtained by following previously procedures,8 using a Nicolet R3m/v diffractometer operating in the 8-28 scan mode with graphitemonochromated MO-K, radiation (A = 0.71069 A). Empirical absorption corrections were applied to the data using the program PSICOR. Further experimental data are giving in Table 3. The structures were solved using the direct methods program XS9 which revealed the positions of the aluminium and phosphorus atoms. The remainder of the atoms were found using standard difference map techniques. Most, but not all of the hydrogens were visible in the final difference map. Hydrogens were included as fixed atom contributors in the final cycle (C-H = 0.96 A). Details of the refinements are given in Table 4. Bond lengths and angles are given in Tables 1 and 3. Atomic scattering factors and anomalous scattering parameters are as given in ref. 9.
(1) G,H,W’
(2) G,H,cW’
334.38 0.4 x 0.5 x 0.7 Rhombohedral 14.344(4) 16.503(7) 2944(2)
376.40 0.5 x 0.3 x 0.3 Rhombohedral 14.978( 1) 34.195(j) 6643( 1)
R, (obv) 1.13 6 1.34 4.0,45.0 193 1792 1507 1171 F, > 6@0) 70
R,C 1.13 4 1.64 4.0,45.0 193 8171 974 778 Fo ’ 6@0) 80
0.0001 0.0436 0.0532
0.0001 0.0574 0.0601
thank the Petroleum Research Fund and the Milton Fund for financial support. Andrew R. Barron is a 1987-1988 Du Pont Young Faculty Fellow. Acknowledgements-We
REFERENCES 1. A. R. Barron, J. Chem. Sot., Dalton Trans. 1988,3047. 2. C. A. Tolman, Chem. Rev. 1977,77,313. 3. A. Almenningen, L. Fernholt and A. Haaland, J. Organomet. Chem. 1978,145,109.
4. D. C. Bradley, H. Chundzynska, M. M. Factor, D. M. Frigo, M. B. Hursthouse, B. Hussain and L. M. Smith, Polyhedron 1988, 7, 1289. 5. G. A. Anderson, F. R. Forgaard and A. Haaland, Acta Chem. Stand. 1972,26,
1947.
6 J. J. Daly, J. Chem. Sot. 1964, 3799. 7’ T. S. Cameron and B. Dahlen, J. Chem. Sot., Perkin zz 1975, 1737.
8. M. D. Healy, D. A. Wierda and A. R. Barron, Organometallics 1988, 7, 2543. 9. G. M. Sheldrick, SHELXTL.PLUS. Nicolet Corporation, Madison, Wisconsin, U.S.A. (1986).
831-834,
1989 0
0277-5387/X9 %3.00+.00 1989 Pergamon Press
plc
ADDUCTS OF TRIMETHYLALUMINIUM WITH PHOSPHINE LIGANDS: X-RAY CRYSTAL STRUCTURES OF Me3AlPPh3 AND Me,AlP(o-tolyl), DERK A. WIERDA and ANDREW R. BARRON* Department of Chemistry, Harvard University, Cambridge, MA 02138, U.S.A. (Received 3 1 August 1988 ; accepted 24 October 1988) Abstract-The
crystal and molecular structures of two phosphine adducts of trimethylaluminium are reported. Compound 1, Me3AlPPh3, crystallizes in the rhombohedral space group R,(obv), a = b = 14.344(4), c = 16.503(7) A, I’= 2944(2) A3, 2 = 6, R = 0.0436, R, = 0.0532. Compound 2, Me,AlP(o-tolyl),, crystallizes in the rhombohedral space group R3C, a = b = 14.978(l), c = 34.195(5) A, V = 6643(l) A3, 2 = 4, R = 0.0574, R, = 0.0601. Structural features are discussed with reference to the 13C and 31P NMR spectroscopic data.
We have recently shown’ that the dominant factor influencing the NMR chemical shifts in Me3AlPR3 complexes is steric repulsion between the aluminium methyl groups and the phosphine substituents. The 13C chemical shifts of the A1-CH3 groups may be used as an indication of the distortion from an idealized planar, monomeric AlMe,, and the change in 3‘P chemical shift (A)? of the phosphines on coordination to AlMe, correlates with the difference in R-P-R angles between free and coordinated phosphine. The complex, Me3A1P(o-tolyl),, does not fit the trends found in 3‘P A or Al-CH, r3C chemical shifts. Based on a calculated cone angle for P(otolyl)32 of 194” a negative 3’P A is expected if the angle R-P-R decreases on coordination. The observed value, however, is positive (+ 7.7 ppm)’ indicating that the C-P-C angle is opened on coordination. This observation suggests that the P(o-tolyl), ligand adopts a conformation so as to relieve steric strain between the o-tolyl and aluminium methyl groups. In order to test the validity of the NMR data as a structural probe for Me3A1PR3 complexes we have undertaken an X-ray crystallographic study of Me,AlPPh, and Me3A1P(o-tolyl),.
RESULTS
AND DISCUSSION
Me3A1PPh3 The molecular structure of Me3AlPPh3 (1) is shown in Fig. 1 and selected bond lengths are given in Table 1. The complex contains a crystallographically-imposed C3 axis coincident with the Al-P bond. The aromatic rings of the triphenylphosphine ligand adopt a propeller-like conformation, with the phenyl groups pitched 38.5” from the plane defined by Al, P and C(1). As a consequence of this the Al-C and P-C bonds are only partly staggered with a dihedral angle of 39.8”.
*Author to whom correspondence should be addressed. t 3‘P A = &comp~cx - &re+osp,,ine.
Fig. 1. X-ray crystal structure of Me,AIPPh, (1) showing the numbering scheme.
831
D. A. WIERDA
832
and A. R. BARRON
Table 1. Selected bond lengths (A) and angles (“) for 1 2.535( 1) 1.820(2) 1.385(3) 1.395(3)
Al-P P--c(l) c(2F(3) C(5)--c(6) Al-P-C( 1) P--c(lHX2) c(ltc(2)--c(3) C(3)-C(4)----c(5) C(5)-C(6)----c(l) The shortest phenyl ring
Al-C( 10) C(lW(2) C(3)--c(4) C(6HYl)
114.0(l) 122.7( 1) 120.1(2) 119.8(2) 120.1(2)
non-bonding distances between and alnmininm methyl group
1.981(2) 1.391(3) 1.377(3) 1.394(2)
P-AI-C( 10) P---U)-C(6) C(2tc(3)-c(4) C(4)_-c(5)--c(6)
the are
C(2)--C(lOb) = 3.819(2) 8, and C(2)-C(lOA) = 3.993(2) A. These contacts are emphasized by viewing the molecule along Al-P (Fig. 2). The Al-P distance, 2.535(l) A, does not vary significantly from that found in Me,AlPMe3 (2.53(4) A)3 or (Me3A1),*(Ph,PCHJ2 (2.544(4) A).” A comparison of the structure of 1 and free monomeric A1Me35 (see Table 2) shows that the lengths of the AI-C bonds increase, and that the
100.6( 1) 119.0(2) 120.8(2) 120.2(2)
C-Al-C angles decrease on coordination. Both changes are in the direction predicted by an increased aluminium p-character in the Al-C bonds on changing from a planar to tetrahedral geometry. From NMR results’ the aluminium centre should be distorted further from planarity in Me,AlPPh3 than in Me3A1PMe3, this is indeed the case. The changes in PPh3 geometry6 on coordination are not so marked (see Table 2). The only appreciable change is an opening of the C-P-C angle by ca 1.6”. This is in contrast to the results from 31P NMR spectroscopy’ which predict that the C-P-C angle is reduced slightly on coordination. It is likely that this discrepancy between solid state and solution is a consequence of the distortions observed in the crystal structure of PPh 3.6 The unit cell of Me3AlPPh3 is comprised of six molecules with no unduly close intermolecular contacts.
Me,AlP(o-tolyl),
Fig. 2. The structure of Me,AlPPh3 (1)viewed down the Al-P vector.
The molecular structure of Me,A1P(o-tolyl)3 (2) is shown in Fig. 3 ; selected bond lengths and angles are given in Table 3. The complex contains a crys-
Table 2. Selected bond lengths (A) and angles (“) of AlMe,, Me,AIPRj phosphines, PR, Al-P
Compound Me,Al” Me,AIPMe,b PMe3’ Me,AlPPh, PPh, Me,AlP(o-tolyl), P(o-tolyl),d a Reference b Reference ’ Reference ‘Reference
5. 3. 6. 7.
Al-C
2.53
1.957 1.973
2.535
1.981
2.584
1.874
P-C
1.822 1.846 1.820 1.828 (av) 1.834 1.86 (av)
complexes and free
C-Al-C 120.0 117.1 116.6 113.9
C-P-C
103.4 97.54 104.6 102.9 (av) 105.1 102.8 (av)
Adducts of trimethylaluminium
with phosphine ligands
833
Table 3. Selected bond lengths (A) and angles (“) for 2 Al-C( 10) P-C(l) C(l>--c(6) C(3)--C(4) CW-C(6) C( lo)-Al-P P--c(l)-C(2) C(2)-C(l)--c(6) C(2)-C(3)--c(4) C(4)-C(5)--c(6) C(l)--c(6)-C(7)
1.974(6) 1.834(4) 1.400(5) 1.376(6) 1.377(7)
Al-P C(l)-C(2) C(2)-~(3) C(4)-W C(6)-C(7)
104.5(l) 119.3(2) 119.7(4) 120.1(5) 122.6(4) 123.1(4)
Fig. 3. X-ray crystal structure of Me,AlP(o-tolyl), showing the numbering scheme.
2.584(2) 1.402(6) 1.384(7) 1.381(9) 1.512(7)
Al-P-C( 1) P--c(l)-C(6) C(~4x2+C(3) C(3)--c(4)-C(5) C(l)--c(6)-C(5) C(5>--c(6)--c(7)
(2)
C3 axis along the Al-P bond. The aromatic rings of the o-tolyl groups adopt a propeller-like arrangement, with a pitch of 50.2” from the plane defined by Al, P and C(1). As a consequence of this, the Al-C and P-C bonds are nearly eclipsed, with a dihedral angle of 10.5”. This conformation allows the o-tolyl methyl groups to be staggered with respect to the altinium methyl groups (Fig. 4). The shortest non-bonding distances between the o-tolyl methyl and the aluminium methyl groups are C(7)--C( 10) = 3.48 l(4) A tallographically-imposed
113.6(l) 120.9(3) 120.3(3) 119.2(5) 118.1(4) 118.7(3)
and C(7)-C(10) = 3.872(5) A. The Al-P distance, 2..584(2) A, is significantly longer than Al-P distances previously reported. 3,4 The changes in the Al-C bond distance and C-Al-C angle on coordination of AlMe, to P(otolyl), (see Table 2) are consistent with increased aluminium p-character in the Al-C bonds on changing from a planar to tetrahedral geometry. From 13C NMR data it was predicted’ that the C-Al-C angle for 2 should be smaller than that found for 1; this is observed (Table 2). The 31P A value for 2 ’ suggested a larger C-P-C angle on coordination than in free phosphine,7 which is confirmed by the crystal structure (see Table 2). It is of course possible that the conformation of the o-tolyl rings observed in the crystal structure is not retained in solution. It is clear, however, that changes in the geometry of 2 predicted by NMR are consistent with those observed in the solid state. The unit cell of Me,AlP(o-tolyl), is comprised of four molecules with no unduly close intermolecular contacts. All the intermolecular distances correspond to, or are greater than, normal van der Waals contacts.
CONCLUSIONS The X-ray structural data reported here confirm that ’3C and 31P NMR data can be used as an indication of changes in geometry around aluminium and phosphorus on coordination of phosphines to AlMe,. There is no evidence, however, that NMR can be used to predict either the Al-P bond length, or the energy of dissociation of the complexes Me3AlPR3. EXPERIMENTAL Fig. 4. The structure of Me,AlP(o-tolyl), the Al-P vector.
(2) viewed down
Crystals were prepared as described in ref. 1 and sealed under nitrogen in Lindemann capillaries.
834
D. A. WIERDA
and A. R. BARRON
Table 4. Crystal data, details of intensity measurement and structure refinement for Me,AlPPh, (1) and Me,AlP(o-tolyl), (2) Compound Formula M, Crystal dimensions (mm) Crystal system a, b (A) c (A) v (A’) Space group
0, (g cm-‘) Z 1 (cm- ‘)
Minimum, maximum (“) 7’ (IQ Total data Total unique data Total observed data Significance test Number of parameters Weighting scheme Parameter g in l/[(F,)*+gF~] Final R Final R,
Unit-cell parameters and intensity data were detailed obtained by following previously procedures,8 using a Nicolet R3m/v diffractometer operating in the 8-28 scan mode with graphitemonochromated MO-K, radiation (A = 0.71069 A). Empirical absorption corrections were applied to the data using the program PSICOR. Further experimental data are giving in Table 3. The structures were solved using the direct methods program XS9 which revealed the positions of the aluminium and phosphorus atoms. The remainder of the atoms were found using standard difference map techniques. Most, but not all of the hydrogens were visible in the final difference map. Hydrogens were included as fixed atom contributors in the final cycle (C-H = 0.96 A). Details of the refinements are given in Table 4. Bond lengths and angles are given in Tables 1 and 3. Atomic scattering factors and anomalous scattering parameters are as given in ref. 9.
(1) G,H,W’
(2) G,H,cW’
334.38 0.4 x 0.5 x 0.7 Rhombohedral 14.344(4) 16.503(7) 2944(2)
376.40 0.5 x 0.3 x 0.3 Rhombohedral 14.978( 1) 34.195(j) 6643( 1)
R, (obv) 1.13 6 1.34 4.0,45.0 193 1792 1507 1171 F, > 6@0) 70
R,C 1.13 4 1.64 4.0,45.0 193 8171 974 778 Fo ’ 6@0) 80
0.0001 0.0436 0.0532
0.0001 0.0574 0.0601
thank the Petroleum Research Fund and the Milton Fund for financial support. Andrew R. Barron is a 1987-1988 Du Pont Young Faculty Fellow. Acknowledgements-We
REFERENCES 1. A. R. Barron, J. Chem. Sot., Dalton Trans. 1988,3047. 2. C. A. Tolman, Chem. Rev. 1977,77,313. 3. A. Almenningen, L. Fernholt and A. Haaland, J. Organomet. Chem. 1978,145,109.
4. D. C. Bradley, H. Chundzynska, M. M. Factor, D. M. Frigo, M. B. Hursthouse, B. Hussain and L. M. Smith, Polyhedron 1988, 7, 1289. 5. G. A. Anderson, F. R. Forgaard and A. Haaland, Acta Chem. Stand. 1972,26,
1947.
6 J. J. Daly, J. Chem. Sot. 1964, 3799. 7’ T. S. Cameron and B. Dahlen, J. Chem. Sot., Perkin zz 1975, 1737.
8. M. D. Healy, D. A. Wierda and A. R. Barron, Organometallics 1988, 7, 2543. 9. G. M. Sheldrick, SHELXTL.PLUS. Nicolet Corporation, Madison, Wisconsin, U.S.A. (1986).