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Tetrahedral or trigonal bipyramidal phosphoranyl radicals
‘yohrme40, nurrtber2
CHEMkAi
_
PHYSICS LETTER’;
1 June !976
. ._
.-
TETRAHEl+iL
..
OR TRiGONAL
BIPtiMIDAL
iHOSPHOR.ANYL
RADICALS
Mgrtyn CR. SYMONS Depnrtmetit
Of
dernkt?y.
The
Lhiversity,
Leicestt!r,
LEI
7RH,
UK
Received 30 January 19k Revised manuscript received 3 hfarch 1976
Although there is good evidence that phosphoranyl radicals with two halide ligands, such as sPF4 have trigonal bipyratidal structures with the unpaired electron formally occupying one equatorial site, certain monohalides are shown to be better described as tet&edral, with the unpaired electron in the U* P-Hal bond.
It has recently been shown [ I,2 J that in the particular case of dichloro-phosphoranyl radicals (specifically,~~_POCl~ [I ] and Cl,l!‘(OR)2 [Z]) structure I is adopted. The unpaired electron is in an orbital having some 3p, character on phosphorus and considerable ox
character on the two chlorine Iigands. This conclusion has been nicely confirmed by studies of lPFh in a single crystal of F’F3 [3], which show clearly that the maximum hyperfine coupling to the two equ$aIent axial fluorine ligands occurs at the minimum P coup’hng and vice versa, as required by I. When an electron is accepted by a closed-shell molecule, some distortion must occur. There are many results for electronexcess radicals which establish that distortion by bond-bending is frequently less favourable than distortion by bond-stretching. Of course, for a molecule such as Clz, there is no alter-
native to bond-stretching to give Cl?, but for a molecule such a linear BrCN, bending to give II or stretching to give III are both reasonable.
N-C-bBr-
I
II
III
We have shown that III is favoured at 77 K [4]_,piespite the fact that the isostructural radicals CICO and Fe0
have structure
226
.--
.-
II [5,6] _ Similarly, radical
--_ :‘
._
anions of the type RCONHHa 1 - prefer to stretch the N-Hal bond to accept the excess electron, rather than becoming pyramidal [7]. We have recently found that a range of mono-chloro and bromo phosphoranyl radicals, formed by electron or halogen atom addition have ESR spectra that conform more c!osely with the cr.+formulation, IV or V, than with I.
A good example, shown in fig. 1, is for the radical (MeO)2P(S)Br- formed by y-radiolysis of (Me0)2P (S)Er. The spectra show definitively that the maximum (parallel) bromine coupling occurs close to the
maximum phosphorus coupling, and both perpendicular couplings also coincide. Although powder spectra do not establish that the two hyperfme tensors are strictly coaxial, we can fmly conclude that they must have very similar directions since both are axial, and none of the peculiarities associated with powdePspectra [8] for species containing two nuclei
having widely different magnetic axes is osberved. Furthermore, the derived data (table I) aie typical of 31P and a1 Br in such a’situation, which im$.ies that both sets represent the principal values, which can only be true if they share common axes.
CHEMlCAL PHYSICS LETTERS
Volume 49. number 2
I
June 1976
,I< .)312Gi
Fig;. 1. First derivative X-band ESR spectrum for
(Me012P(S)Brafter %Josure to 6oCo r-rays at 77 K, showing features assigned
to (Me0)2P(S)Br- radicals. These
considerations
are strongly
supported
by
results for irradiated single crystals of triphenylphosphiie-borontrichloride. Berclaz et al. [9] have recently Table 1 Hypertine coupling constants derived from the ESR spectrum for (Me0)2P(S)Br-, together with calculated orbital popuiations
hyperfme coupling (G)a)
orbital populations c,
A 11 Al A-!SO o&d) aZp
3’P
*lBr
780 600
340 100
660 0.18 0.59
Iso’-‘) 0.02 0.28
=)lG=104T. b) Positive signs are assumed: if Al is taken- to be negative, the anisotropic coupling is unreasonaMy large. c) Derived using A0 and 2B” values given in ref. [14]. d),=3for31~,nn4fora1Br.
shown that a secondary radical formed at room temperature contains one phosphorus and one chlorine atom, and have postulated the species Ph,kl. We have also studied i&is material [lo] and confirm the formation of this species. The important result is that the principal directions of the chlorine and phosphorus hyperfme tensors are nearly the same [IO], and this has lead Berclaz et al. [9] to the conclusion that the excess electron is ‘ZIthe P-Cl o* oibital. We conclude that two limiting structures, I and IV, are possible for phosphoranyl radicals. I will be favoured for lP& and -PR2x2 radic&s, or -PRX, radicals if X is more electronegative than R (cfz -PH4 has structure I [ 1 I]). However, v will be favoured for lPR3X radicals. Some radicals of low symmetry may have structures intermediate between these simple forms. The gPR3X CJ*radicks may welt experience a flattening of the R3P- group, but are not 227
.,
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,-.’
.,.r
;
_.,.
i
c
-:.,
_
_.
‘.
I
.’
.
l&dy.t3&i&&ii fhi &&iarlimit
(VJ
sinceeven-in
__ .ihe limit~~f R3Pf.fo&natioon? the group r&&s : .’ &y&j& _-‘- -. ._ ,_These considkr~tions~c&r p&ably be &neralied -to&her rA.ic& of thhis.type. Thus, for‘exantple, we . ’ hi& &&I that ph3pb-Hdradkals have a structure do.& to‘IV than I [12]. :. p+n a second electron is added, radicals of struci ture I transfobnn_without structural chtige into the normal “pentacovalent” molecules, : PP,-.+ Hal-. Fiially, it is worth recalling that the:“dimer’* radicals RsP-~PR;, formed by iiradiated phosphines [ 131 have two equivalent phosphorus atoms, &rich means that they must have the u* itructure,IV rather than the unsymiet&al structure I, although I is a perfectly reasonable aiternative. It seems clear that the energy differiericesbetween these limiting structures must -generally be small.
References 121 T. GiUbroand F. Wfiams, 5.
Am. Chem.
Sac.
.i2]- D.J. Nelson aal;i M.C.R. Symbns, j.Chem. Sot. Dalton 1. _- (!975)1164. {3j. 4. Ebsegawa, K. Oh&hi, K. Sogabe and %. Miura, . Mol. Phys. 30 (1975) 1367. [hi S-P. Mishra, G.W. Nei!son and M.C.R.S&nons, J. Chem. Sot. Fz%iay I1 70 (1974) 1280. [S] F.J. Adrian, E.L. Co&an and V.A. 15owers. J. Chem. ?hys. 56 (1972) 6251. (61 F.J. Adrian, E.L Co&ran and V.A. Bowers, J. Chem. Phys. 44 (1966) 4626: [71 G.W. Neilsnn and M.C.K.Srmons. J. Chem. Sac. Faraday II 68.(1972) 1582; Mol. Phys. 27 (1974) 1613. f81 M.C.R. Symons..D.X. West and J-G. Wilkinson, J. Chem. Sr?c. Dalton (1974) 2247. [91 T. Berchz, M. @offroy and E.A.C. r,ucken, Chem. Phys. Letters 36 (1975) 677. [lOI J. Drake and M-CR. Symons, unpublished results. [IfI A.J. Colussi, J-R. Morton and K.F. Preston, J. Chem. Phys. 62 (1975) 2004. iI21 O.P. Anderson, S.X. Fieldhobse, H.C. Starkie and M.C.R. Symons, Mol. Phys. 26 (1973) 1.561. (131 A.R. Lyons and M.C.R. Symons, J. Chem. Sot. Faraday II 68 (1972) 1589. I141 P.W. Atkins and M.C.R. Symons, The structure of inorganic Iadicals (Elsevies, Amsterdam, 1967).
96
(1974) 5032.
_.
._‘-
:
__’
.._
-.
.__^
._
..
.
__
:
‘-
CHEMkAi
_
PHYSICS LETTER’;
1 June !976
. ._
.-
TETRAHEl+iL
..
OR TRiGONAL
BIPtiMIDAL
iHOSPHOR.ANYL
RADICALS
Mgrtyn CR. SYMONS Depnrtmetit
Of
dernkt?y.
The
Lhiversity,
Leicestt!r,
LEI
7RH,
UK
Received 30 January 19k Revised manuscript received 3 hfarch 1976
Although there is good evidence that phosphoranyl radicals with two halide ligands, such as sPF4 have trigonal bipyratidal structures with the unpaired electron formally occupying one equatorial site, certain monohalides are shown to be better described as tet&edral, with the unpaired electron in the U* P-Hal bond.
It has recently been shown [ I,2 J that in the particular case of dichloro-phosphoranyl radicals (specifically,~~_POCl~ [I ] and Cl,l!‘(OR)2 [Z]) structure I is adopted. The unpaired electron is in an orbital having some 3p, character on phosphorus and considerable ox
character on the two chlorine Iigands. This conclusion has been nicely confirmed by studies of lPFh in a single crystal of F’F3 [3], which show clearly that the maximum hyperfine coupling to the two equ$aIent axial fluorine ligands occurs at the minimum P coup’hng and vice versa, as required by I. When an electron is accepted by a closed-shell molecule, some distortion must occur. There are many results for electronexcess radicals which establish that distortion by bond-bending is frequently less favourable than distortion by bond-stretching. Of course, for a molecule such as Clz, there is no alter-
native to bond-stretching to give Cl?, but for a molecule such a linear BrCN, bending to give II or stretching to give III are both reasonable.
N-C-bBr-
I
II
III
We have shown that III is favoured at 77 K [4]_,piespite the fact that the isostructural radicals CICO and Fe0
have structure
226
.--
.-
II [5,6] _ Similarly, radical
--_ :‘
._
anions of the type RCONHHa 1 - prefer to stretch the N-Hal bond to accept the excess electron, rather than becoming pyramidal [7]. We have recently found that a range of mono-chloro and bromo phosphoranyl radicals, formed by electron or halogen atom addition have ESR spectra that conform more c!osely with the cr.+formulation, IV or V, than with I.
A good example, shown in fig. 1, is for the radical (MeO)2P(S)Br- formed by y-radiolysis of (Me0)2P (S)Er. The spectra show definitively that the maximum (parallel) bromine coupling occurs close to the
maximum phosphorus coupling, and both perpendicular couplings also coincide. Although powder spectra do not establish that the two hyperfme tensors are strictly coaxial, we can fmly conclude that they must have very similar directions since both are axial, and none of the peculiarities associated with powdePspectra [8] for species containing two nuclei
having widely different magnetic axes is osberved. Furthermore, the derived data (table I) aie typical of 31P and a1 Br in such a’situation, which im$.ies that both sets represent the principal values, which can only be true if they share common axes.
CHEMlCAL PHYSICS LETTERS
Volume 49. number 2
I
June 1976
,I< .)312Gi
Fig;. 1. First derivative X-band ESR spectrum for
(Me012P(S)Brafter %Josure to 6oCo r-rays at 77 K, showing features assigned
to (Me0)2P(S)Br- radicals. These
considerations
are strongly
supported
by
results for irradiated single crystals of triphenylphosphiie-borontrichloride. Berclaz et al. [9] have recently Table 1 Hypertine coupling constants derived from the ESR spectrum for (Me0)2P(S)Br-, together with calculated orbital popuiations
hyperfme coupling (G)a)
orbital populations c,
A 11 Al A-!SO o&d) aZp
3’P
*lBr
780 600
340 100
660 0.18 0.59
Iso’-‘) 0.02 0.28
=)lG=104T. b) Positive signs are assumed: if Al is taken- to be negative, the anisotropic coupling is unreasonaMy large. c) Derived using A0 and 2B” values given in ref. [14]. d),=3for31~,nn4fora1Br.
shown that a secondary radical formed at room temperature contains one phosphorus and one chlorine atom, and have postulated the species Ph,kl. We have also studied i&is material [lo] and confirm the formation of this species. The important result is that the principal directions of the chlorine and phosphorus hyperfme tensors are nearly the same [IO], and this has lead Berclaz et al. [9] to the conclusion that the excess electron is ‘ZIthe P-Cl o* oibital. We conclude that two limiting structures, I and IV, are possible for phosphoranyl radicals. I will be favoured for lP& and -PR2x2 radic&s, or -PRX, radicals if X is more electronegative than R (cfz -PH4 has structure I [ 1 I]). However, v will be favoured for lPR3X radicals. Some radicals of low symmetry may have structures intermediate between these simple forms. The gPR3X CJ*radicks may welt experience a flattening of the R3P- group, but are not 227
.,
-:
,-.’
.,.r
;
_.,.
i
c
-:.,
_
_.
‘.
I
.’
.
l&dy.t3&i&&ii fhi &&iarlimit
(VJ
sinceeven-in
__ .ihe limit~~f R3Pf.fo&natioon? the group r&&s : .’ &y&j& _-‘- -. ._ ,_These considkr~tions~c&r p&ably be &neralied -to&her rA.ic& of thhis.type. Thus, for‘exantple, we . ’ hi& &&I that ph3pb-Hdradkals have a structure do.& to‘IV than I [12]. :. p+n a second electron is added, radicals of struci ture I transfobnn_without structural chtige into the normal “pentacovalent” molecules, : PP,-.+ Hal-. Fiially, it is worth recalling that the:“dimer’* radicals RsP-~PR;, formed by iiradiated phosphines [ 131 have two equivalent phosphorus atoms, &rich means that they must have the u* itructure,IV rather than the unsymiet&al structure I, although I is a perfectly reasonable aiternative. It seems clear that the energy differiericesbetween these limiting structures must -generally be small.
References 121 T. GiUbroand F. Wfiams, 5.
Am. Chem.
Sac.
.i2]- D.J. Nelson aal;i M.C.R. Symbns, j.Chem. Sot. Dalton 1. _- (!975)1164. {3j. 4. Ebsegawa, K. Oh&hi, K. Sogabe and %. Miura, . Mol. Phys. 30 (1975) 1367. [hi S-P. Mishra, G.W. Nei!son and M.C.R.S&nons, J. Chem. Sot. Fz%iay I1 70 (1974) 1280. [S] F.J. Adrian, E.L. Co&an and V.A. 15owers. J. Chem. ?hys. 56 (1972) 6251. (61 F.J. Adrian, E.L Co&ran and V.A. Bowers, J. Chem. Phys. 44 (1966) 4626: [71 G.W. Neilsnn and M.C.K.Srmons. J. Chem. Sac. Faraday II 68.(1972) 1582; Mol. Phys. 27 (1974) 1613. f81 M.C.R. Symons..D.X. West and J-G. Wilkinson, J. Chem. Sr?c. Dalton (1974) 2247. [91 T. Berchz, M. @offroy and E.A.C. r,ucken, Chem. Phys. Letters 36 (1975) 677. [lOI J. Drake and M-CR. Symons, unpublished results. [IfI A.J. Colussi, J-R. Morton and K.F. Preston, J. Chem. Phys. 62 (1975) 2004. iI21 O.P. Anderson, S.X. Fieldhobse, H.C. Starkie and M.C.R. Symons, Mol. Phys. 26 (1973) 1.561. (131 A.R. Lyons and M.C.R. Symons, J. Chem. Sot. Faraday II 68 (1972) 1589. I141 P.W. Atkins and M.C.R. Symons, The structure of inorganic Iadicals (Elsevies, Amsterdam, 1967).
96
(1974) 5032.
_.
._‘-
:
__’
.._
-.
.__^
._
..
.
__
:
‘-