Complexes of the lanthanide metals (La-Nd, Sm-Lu) with hypophosphite and phosphite ligands: Crystal structures of [Ce(H2PO2)3(H2O)], [Dy(H2PO2)3] and [Pr(H2PO2)(HPO3)(H2O)]·H 2O

Article abstract of DOI:10.1039/a807727d

Reactions of lanthanide chlorides LnCl₃ with NaH₂PO₂ in aqueous solution afforded a series of complexes [Ln(H₂PO₂)₃(H₂O)_n] (Ln = La-Tb, n = 1; Ce-Nd, Dy-Lu; n = 0) with members falling into at least four separate structural types. The crystal structures of members of two of these types, [Ce(H₂PO₂)₃(H₂O)] and [Dy(H₂PO₂)₃], have been determined for the first time, along with that of [Pr(H₂PO₂)(HPO₃)(H₂O)]·H ₂O, one of a new series of complexes [Ln(H₂PO₂)(HPO₃)(H₂O) _n]·nH₂O (Ln = La-Gd, n = 1; Tb-Lu; n = 0) obtained by controlled oxidation of [Ln(H₂PO₂)₃(H₂O)_n]. The hydrated members of this series are notable due to their incorporation of chiral channels inhabited by water molecules, and to their ability reversibly to dehydrate. Steric angle sum calculations have been used to rationalise the morphologies of the new complexes.

Full text of DOI:10.1039/a807727d

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Complexes of the lanthanide metals (La–Nd, Sm–Lu) with
hypophosphite and phosphite ligands: crystal structures of
[Ce(H2PO2)3(H2O)], [Dy(H2PO2)3] and [Pr(H2PO2)(HPO3)-
(H2O)]ؒH2O
Joanne A. Seddon, Andrew R. W. Jackson, Roman A. Kresin´ski* and Andrew W. G. Platt
School of Sciences, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire,
UK ST4 2DE. E-mail: sctrak@staffs.ac.uk
Received 5th October 1998, Accepted 29th April 1999
Reactions of lanthanide chlorides LnCl3 with NaH2PO2 in aqueous solution afforded a series of complexes
[Ln(H2PO2)3(H2O)n] (Ln = La–Tb, n = 1; Ce–Nd, Dy–Lu; n = 0) with members falling into at least four separate
structural types. The crystal structures of members of two of these types, [Ce(H2PO2)3(H2O)] and [Dy(H2PO2)3],
have been determined for the first time, along with that of [Pr(H2PO2)(HPO3)(H2O)]ؒH2O, one of a new series of
complexes [Ln(H2PO2)(HPO3)(H2O)n]ؒnH2O (Ln = La–Gd, n = 1; Tb–Lu; n = 0) obtained by controlled oxidation of
[Ln(H2PO2)3(H2O)n]. The hydrated members of this series are notable due to their incorporation of chiral channels
inhabited by water molecules, and to their ability reversibly to dehydrate. Steric angle sum calculations have been used
to rationalise the morphologies of the new complexes.
Tripositive lanthanide ions are influenced only slightly by their
own d or f orbitals in their bonding with ligands, these inter-
actions being largely electrostatic. Others1 have expounded
from this that the concept of co-ordination saturation becomes
equated with one of steric saturation in the case of lanthanide
complexes. Thus, a wide variety of physicochemical properties
of lanthanide complexes can vary smoothly when surveyed
across the lanthanide series, until, at a certain limit, an abrupt
variation is encountered.2 This phenomenon is ascribable to the
ionic radius of the lanthanide metal decreasing smoothly up
to a point where the metal environment is sufficiently crowded
as to eject one or more ligand atoms from the primary
co-ordination sphere.
In a desire to survey systematically the structures of a
family of simple lanthanide ion complexes we sought a ligand
system which was uncomplicated, but nonetheless relatively
unexplored, and this prompted our study of hypophosphite
(phosphinate) complexes of lanthanides. Hypophosphite-
containing structures appear to be well distributed around the
Periodic Table: structures reported to date include those of
d-block,3–5 s-block,6–9 p-block,10,11 and actinide metals,12,13 and
an early structure of hypophosphite as its ammonium salt.14
These show the hypophosphite ion to be very flexible in its
co-ordination, doubly bridging11,13 µ(κ1,κ1), triply bridging3,5
µ3(κ1,κ1,κ1), and quadruply bridging4,6 µ4(κ1,κ1,κ1,κ1) modes all
being represented.
denoted by us here as A and AЈ respectively, accommodate a
water ligand in their co-ordination spheres.15,16 Two of the
hypophosphite ligands adopt the µ(κ1,κ1) mode and the third
µ3(κ1,κ1,κ1), the co-ordination number thus being eight in both
structures. Despite their evident close similarity, the A and AЈ
structures differ in the disposition of the µ3(κ1,κ1,κ1) ligand, and
therefore cannot be considered isostructural.16 The transition
point between them in the lanthanide series has not yet been
located and, in planning so to do, we considered that the close
similarity of these structural types may prohibit unambiguous
attribution of class to intervening lanthanide analogues simply
by means of spectroscopic techniques. We anticipated that
powder X-ray diffraction and, given the ability to grow suitable
crystals, single-crystal X-ray diffraction, would prove our most
valuable tools in achieving this and our other aims, which also
include establishing for which metals anhydrous structural
types may be formed. An example is the recently discovered17
structural type B adopted by [Pr(H2PO2)3], which achieves 8-co-
ordination by use of the µ(κ1,κ1) mode for one ligand only, the
other two being µ3(κ1,κ1,κ1). In contrast, the late lanthanide
complex [Er(H2PO2)3] is reported18 to be 6-co-ordinate, having
an approximately octahedral geometry about the metal ion, all
hypophosphite ligands adopting the µ(κ1,κ1) mode; this struc-
ture is denoted by us here as belonging to type C. Our final aim
was to explore whether other lanthanide structures might adopt
types B, C, or an entirely new structural form, potentially one
adopting the comparatively rare1 7-co-ordination.
H
H
H
H
H
H
P
P
P
Results and discussion
Synthetic studies
O
O
O
O
Ln
Ln
O
O
Ln
Ln
Ln
Ln
Ln
Ln
Ln
The reaction of aqueous solutions of lanthanide chlorides with
sodium hypophosphite gave rise to the corresponding lan-
thanide hypophosphites. The compounds are all poorly soluble
in water, but dissolve in dilute HCl. Their spectroscopic and
other properties, and subsequent structural studies (discussed
below), showed that the compounds form four distinct series of
structural types depending on the metal and the reaction condi-
tions. These and their interconversions are summarised in Table
1. In some cases air oxidation results in the formation of
µ(κ1, κ1)
µ3(κ1, κ1, κ1)
µ4(κ1, κ1, κ1, κ1)
Four structures of lanthanide hypophosphite complexes have
been elucidated to date. These are consistent with the expect-
ation that later lanthanide ions should be less able to accom-
modate high co-ordination numbers than the larger, earlier
lanthanides. Thus, the closely related complexes [La(H2PO2)3-
(H2O)] and [Eu(H2PO2)3(H2O)], belonging to structural types
J. Chem. Soc., Dalton Trans., 1999, 2189–2196
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Table 1 Formation of the various classes of lanthanide hypophosphite complexes
La
Ce
Pr
Nd (Pm)
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
A
AЈ
B
Ί
Ί
Ί
Ί
Ί
Ί
Ί
Ί
b
b
b
C
c
c
Ί
Ί
Ί
Ί
Ί
Ί
Ί, Formed on mixing reagents; b, formed from type A by digestive dehydration; c, formed on forced dehydration of type AЈ.
phosphite, manifest in the 31P NMR spectra of the praseo-
dymium and neodymium products completely dissolved in
aqueous HCl. This oxidation is suppressed by carrying out the
reaction under nitrogen or by buffering the solution to pH 1.4.
Digestion of the type A complexes on a water-bath over one
day leads to the formation of anhydrous type B complexes for
Ce, Pr and Nd. Interestingly no type B complex is observed
for lanthanum and the type AЈ compounds do not undergo
similar reactions. On prolonged digestion open to the air well
defined mixed hypophosphite–phosphite complexes are
formed. Two distinct forms of the mixed complex are observed,
type 1, [Ln(H2PO2)(HPO3)(H2O)]ؒH2O (Ln = Pr–Gd) and an
anhydrous type 2, [Ln(H2PO2)(HPO3)] (Ln = Tb–Lu), but we
have never unambiguously identified the lanthanum and cerium
type 1 analogues. Over prolonged periods evidence of further
oxidation to phosphate is observed in the 31P NMR spectra of
2Ϫ
solutions of the products which show, in addition to HPO3
3Ϫ
and H2PO2Ϫ, a singlet due to PO4
.
The substitution of lanthanide chlorides by the nitrates,
whilst giving the same hypophosphite complexes, leads to the
acceleration of the formation of type 1 and 2 materials; in these
cases the oxidation of hypophosphite by nitrate was confirmed
by the identification of the main gaseous nitrogen containing
product as N2O contaminated with smaller amounts of NO, the
infrared spectrum of the gas collected from the reaction mix-
ture being identical with that expected from the literature.19 Of
note here is the observation that the cerium type B complex
does not easily appear to undergo oxidation under these
conditions.
In view of the formation of anhydrous hypophosphites for
the heavier lanthanides it seemed reasonable to suppose that at
some point the type AЈ and type C structures might possess
similar stabilities and thus be interconvertible. Indeed, reaction
of [Tb(H2PO2)3(H2O)], the last type AЈ complex, with refluxing
HC(OMe)3 gives a type C complex which is considerably more
soluble in water than the other type C compounds and can be
precipitated therefrom as type AЈ by the addition of acetone.
When Ln = Eu a type C compound is also formed, which
rapidly rehydrates in air.
Fig. 1 Typical infrared spectra of all structural types.
two or more different types of hypophosphite. They lose no
significant mass below 310 ЊC at which point an exothermic loss
of 5% mass occurs. This is also attributable to the dispropor-
tionation of hypophosphite and, in common with the cases of
A and AЈ above, is associated with another, concomitant, event,
although much less in evidence here than in A or AЈ. Type C
complexes possess uncomplicated infrared spectra indicative of
rather uniform hypophosphite environments. They lose no
significant mass below 340 ЊC, at which point there is an
exothermic gain in mass of ca. 4% occurring over ca. 40 ЊC.
Presumably this is air oxidation of hypophosphite, no loss of
phosphine being observed; this fact can be rationalised by the
structural studies discussed below. For all the above type A,
AЈ, B and C complexes, their 31P NMR spectra measured in
aqueous HCl show only hypophosphite ion in solution.
Attempts to convert type 1 into type 2 materials by dehydra-
Infrared spectra of the new type 1 complexes show intense
broad bands in the appropriate regions indicating the presence
of water. The P–H region is simple relative to type A complexes,
but the P–O region is rather more complex. Thermal analysis
reveals an endothermic loss of mass of ca. 11% at 130 ЊC, which
corresponds to a loss of two water molecules per metal atom,
followed by no further events up to 460 ЊC. Samples heated to
270 ЊC and allowed to stand in air appear to rehydrate to
regenerate materials with type 1 infrared spectra. Most notably,
however, 31P NMR spectra of types 1 in aqueous HCl indicate
the presence of both phosphite and hypophosphite, suggesting
an empirical formulation of Ln(H2PO2)(HPO3)(H2O)2 for type
1 complexes. Proton coupled 31P NMR spectra of the com-
plexes dissolved in concentrated HCl clearly show the presence
of both phosphite (doublet) and hypophosphite (triplet). The
integrated peak area ratios of these, measured using differing
pulse delays and pulse widths, remains unaltered at 1:1, imply-
ing that rapid relaxation is occurring due to the presence of the
paramagnetic lanthanide ions, and thus the measured ratio is a
good reflection of the relative amounts of phosphite and hypo-
tion with HC(OMe)3 have not been successful.
Spectroscopic and thermal studies
All type A, AЈ, B and C complexes produced possess spectral
and thermal properties consistent with their known15–18 struc-
tures. Thus the types A and AЈ have very similar infrared spec-
tra. Both are indicative of the presence of two or more different
types of hypophosphite environment, and the presence of water
(Fig. 1). Types AЈ have only three P–H stretching bands, pre-
sumably due to overlap. Thermal analysis shows an endotherm
at around 210 ЊC, associated with a loss of ca. 5% mass, and
consistent in all cases with loss of water. At around 310 ЊC there
is a further compound exothermic event, resulting in a further
2% loss of mass; infrared investigation of the gaseous products
of this decomposition revealed that phosphine20 is evolved, and
we believe the change to be attributable to the known21 dispro-
portionation of hypophosphite, occurring concomitantly with
its partial oxidation. Type B complexes possess infrared spectra
indicative of the absence of water ligands and the presence of
2190
J. Chem. Soc., Dalton Trans., 1999, 2189–2196

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Table 2 Crystal data for lanthanide hypophosphite complexes
Formula
M
Crystal system
Space group
a/Å
b/Å
c/Å
[Ce(H2PO2)3(H2O)]
353.09
[Dy(H2PO2)3]
357.46
Monoclinic
C2/m
14.368(3)
5.7340(10)
12.1230(10)
[Pr(H2PO2)(HPO3)(H2O)]ؒH2O
1287.62
Orthorhombic
P212121
6.6558(5)
7.1539(5)
Triclinic
¯
P1
7.1729(10)
7.9827(9)
8.8710(6)
110.643(12)
98.101(13)
104.970(11)
2.642
16.5506(18)
α/Њ
β/Њ
γ/Њ
122.33(2)
Dc/g cmϪ3
Colour
F(000)
Size/mm
θ Range/Њ
hkl ranges
2.813
Lilac
660
2.713
Light green
608
0.25 × 0.28 × 0.30
3.10–25.03
Ϫ7 ≤ h ≤ 7, Ϫ7 ≤ k ≤ 6,
Ϫ18 ≤ l ≤ 17
3274
None
334
0.20 × 0.25 × 0.30
2.54–24.92
Ϫ8 ≤ h ≤ 5, Ϫ9 ≤ k ≤ 8,
Ϫ10 ≤ l ≤ 10
1964
0.35 × 0.24 × 0.24
1.99–24.84
Ϫ16 ≤ h ≤ 16, Ϫ6 ≤ k ≤ 5,
Ϫ10 ≤ l ≤ 13
1816
Total data
Unique data
Merging R
µ/mmϪ1
1274
0.0580
5.661
0.532, 1.000
0.0374
0.0922
1.112
717
0.0495
9.398
0.584, 1.000
0.0309
0.0850
1.129
1220
0.0670
6.574
0.740, 1.000
0.0430
0.1029
1.051
tmin, tmax
R (all data)
wR2 (all data)
Goodness of fit
phosphite present. The spectral properties of type 2 materials
are similar to those of type 1 mentioned above, with the excep-
tion that no bands arising from water are evident in their
infrared spectra. Their thermogravimetric traces are almost fea-
tureless below 460 ЊC. The infrared spectra of the materials
obtained by thermal dehydration of type 1 complexes are very
similar to those of type 2, and we believe type 2 complexes to be
the anhydrous analogues of type 1, the absence of water being
necessitated by smaller ionic radii of the later lanthanides, in
the same way as type C are analogues of type A or AЈ. We
therefore tentatively assign the formulation Ln(H2PO2)(HPO3)
to type 2. There is some evidence here of the existence of alter-
native structural types which might be denoted 1Ј and 2Ј, by
analogy with AЈ. This arises from the interesting fact that type 1
materials when heated to 160 ЊC are completely dehydrated,
and upon rehydration in air revert to type 1. When heated to
270 ЊC, however, they possess different IR spectra after com-
plete rehydration. These transformations are irreversible, but
the products still contain the same 1:1 ratio of hypophosphite
to phosphite (NMR evidence).
Fig. 2 A view22 of [Ce(H2PO2)3(H2O)] along the unit cell b axis. The
Ce, P, O and H atoms are represented by spheres of decreasing relative
arbitrary size. Only the water H atoms are shown, for reasons of clarity,
along with their hydrogen-bonding contacts. Symmetry operators used
throughout: i = Ϫx, Ϫy, Ϫz; ii = x ϩ 1, y, z; iii = 1 Ϫ x, 1 Ϫ y, 1 Ϫ z;
iv = 1 Ϫ x, Ϫy, Ϫz; v = x ϩ 1, y ϩ 1, z; vi = Ϫx, Ϫy, 1 Ϫ z; vii = x, Ϫy, z;
Structural studies
Powder X-ray diffraction patterns were simulated for all of
types A, AЈ, B and C based upon the literature.15–18 Experi-
mental traces were recorded for all the new and known type A,
AЈ, B and C complexes and showed four families of patterns, in
accordance with the categories assigned to each complex by
spectroscopic and thermal means. Good agreement was also
obtained between each pattern prediction and the experimental
trace in all cases, the worst fit being for types C. Patterns were
also recorded for all type 1 materials, and also showed a familial
resemblance. As a final verification of the compositions of these
materials, and in order to clarify the discrepancy between the
predicted and observed powder patterns of type C complexes,
we sought to examine one of each type by single-crystal X-ray
diffraction and found Ce to give satisfactory crystals of type A,
Dy of C, and Pr of types B and 1. No satisfactory crystals were
obtained of any type AЈ complex except for Eu, or of any type 2
complex. Each of the above crystals afforded a structural solu-
tion. Details salient to data collection and structural refinement
are presented in Table 2 (and elsewhere for [Pr(H2PO2)3]).17
The structure of the cerium type A complex was confirmed to
be [Ce(H2PO2)3(H2O)] and is depicted in Fig. 2. It is, as indi-
cated by supporting methods, virtually isostructural15 with
x
viii = x, 1 Ϫ y, z; ix = 2 Ϫ x, y ϩ 2, 2 Ϫ z; = 2 Ϫ x, y Ϫ 2, 2 Ϫ z; xi = x Ϫ 1,
1
1
1
1
–
–
–
–
xii
xiii
xiv
1
1
1
1
1
–
–
–
–
–
y, z; = x Ϫ 2, 12 Ϫ y, Ϫz; = 12 Ϫ x, 1 Ϫ y, z Ϫ 2; = 12 Ϫ x, 2 Ϫ y,
z Ϫ 2; xv = 1 Ϫ x, 1 Ϫ y, Ϫz; xvi = Ϫx, y Ϫ 1, 1 Ϫ z; xvii = x, y Ϫ 1,
1
–
z; xviii = Ϫx, y, 1 Ϫ z; xix = Ϫx, 1 Ϫ y, 1 Ϫ z; xx = Ϫx, y Ϫ 1, Ϫz; xxi = Ϫx,
y, Ϫz; xxii = Ϫx, 1 Ϫ y, Ϫz; xxiii = 12 Ϫ x, 1 Ϫ y, z ϩ 2;
= 12 Ϫ x, 2 Ϫ y,
xxiv
1
1
1
–
–
–
xxv
z ϩ 2; = x ϩ 2, 12 Ϫ y, Ϫz; xxvi = x, y ϩ 1, z.
1
1
1
–
–
–
[La(H2PO2)3(H2O)], the structure of which is outlined briefly
above. The Ln–O (hypophosphite) distance therein is 2.560[7]
i = n
i = 1
Å, but only 2.492[2] Å ([σmean] =
Σ(σi2)/n) in the cerium
analogue (Table 3). There appears to be some compensation for
this contraction in an extension of the hypophosphite ligands;
the P–O (hypophosphite) lengths in [Ce(H2PO2)3(H2O)] average
1.495[2] Å as compared with 1.478[6] Å in the lanthanum
analogue, the O–P–O angles likewise extending to average
117.77[17]Њ as compared to 115.6[5]Њ in [La(H2PO2)3(H2O)]. The
hydrogen atoms of the waters were not locateable, but their
presence was inferred from two, and only two, close contacts
between O(1) and hypophosphite atoms O(21)iv and O(22)v
which subtend almost tetrahedral angles for O(21)iv ؒ ؒ ؒ O(1)–
Ce and O(21)iv ؒ ؒ ؒ O(1) ؒ ؒ ؒ O(22)v. The O(22)v ؒ ؒ ؒ O(1)–Ce
J. Chem. Soc., Dalton Trans., 1999, 2189–2196
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Table 3 Selected interatomic separations (Å) and angles (Њ) for
[Ce(H2PO2)3(H2O)]
Ce–O(11)
Ce–O(31)
2.432(4)
2.416(5)
2.574(4)
2.482(5)
1.483(5)
1.496(5)
1.484(5)
2.714(7)
Ce–O(21)
Ce–O(1)
2.502(5)
2.515(5)
2.607(4)
2.434(5)
1.514(5)
1.504(5)
1.489(5)
2.834(7)
Ce–O(12)i
Ce–O(22)i
P(1)–O(11)
P(2)–O(22)
P(3)–O(31)
O(1) ؒ ؒ ؒ O(21)iv
Ce–O(12)ii
Ce–O(32)iii
P(1)–O(12)
P(2)–O(21)
P(3)–O(32)
O(1) ؒ ؒ ؒ O(22)v
O(31)–Ce–O(11)
86.59(17) O(31)–Ce–O(32)iii
75.64(18)
150.06(18)
75.24(16)
72.10(15)
122.41(15)
146.97(16)
74.52(15)
132.11(16)
143.38(15)
73.06(15)
71.36(15)
119.25(15)
76.11(14)
64.69(16)
119.19(28)
110.75(19)
O(11)–Ce–O(32)iii 79.92(17) O(31)–Ce–O(22)i
O(11)–Ce–O(22)i
O(31)–Ce–O(21)
81.64(16) O(32)iii–Ce–O(22)i
78.99(18) O(11)–Ce–O(21)
O(32)iii–Ce–O(21) 143.03(16) O(22)i–Ce–O(21)
O(31)–Ce–O(1)
O(32)iii–Ce–O(1)
O(21)–Ce–O(1)
102.88(18) O(11)–Ce–O(1)
72.20(17) O(22)i–Ce–O(1)
140.46(15) O(31)–Ce–O(12)i
O(11)–Ce–O(12)i 119.21(15) O(32)iii–Ce–O(12)i
O(22)i–Ce–O(12)ii 77.19(15) O(21)–Ce–O(12)i
Fig. 4 A view22 of [Dy(H2PO2)3]. The Dy, P and O atoms are repre-
sented by spheres of decreasing relative arbitrary size, and H atoms are
omitted.
O(1)–Ce–O(12)i
77.56(15) O(31)–Ce–O(12)ii
O(11)–Ce–O(12)ii 144.17(15) O(32)iii–Ce–O(12)ii
O(22)i–Ce–O(12)ii 130.51(14) O(21)–Ce–O(12)ii
but we note that the veracity of the former has been queried
elsewhere.23 Although the structures do not agree, the axial
lengths of the two unit cells do, only the β angles being differ-
ent. The two structures share the common characteristic of
consisting a framework of metal atoms linked by µ(κ1,κ1)-
hypophosphite bridges extending along the principal axes of
the lattice. The metal atoms (of which there are two crystal-
lographically unique) in [Dy(H2PO2)3] are thereby six-co-
ordinate. The co-ordination geometry is near-octahedral (Fig.
4), the trans angles being perfectly 180Њ and the cis angles ran-
ging close around 90Њ, both extremes of this range being found
at Dy(2). The mean Dy–O distance is 2.253[1] Å (Table 5),
shorter than in any other lanthanide hypophosphite complex
(leaving aside [Er(H2PO2)3]).18
The presence of two waters per metal ion for the Type 1
complexes is substantiated by the emergent structure of [Pr-
(H2PO2)(HPO3)(H2O)]ؒH2O (a formulation to be preferred over
our former interpretation).17 Only one of these waters is associ-
ated directly with the Pr. The metal is once again 8-co-ordinate
by means of additional contributions from µ(κ1,κ1)-hypo-
phosphite and a µ4(κ2,κ1,κ1,κ1)-phosphite (Fig. 5).
O(1)–Ce–O(12)ii
67.61(14) O(12)i–Ce–O(12)ii
O(11)–P(1)–O(12) 115.59(28) O(22)–P(2)–O(21)
O(31)–P(3)–O(32) 118.48(32) Ce–O(1) ؒ ؒ ؒ O(21)iv
Ce–O(1) ؒ ؒ ؒ O(22)v 142.04(22) O(21)iv ؒ ؒ ؒ O(1) ؒ ؒ ؒ O(22)v 107.06(21)
H
O
O
O
Ln
Ln
P
Fig. 3 The co-ordination polyhedron in [Ce(H2PO2)3(H2O)].
Ln
Ln
angle, however, is far larger, which implies that, assuming an sp3
hybridisation at O(1), the attitude of the lone pairs of O(1) to
Ce is not one of direct alignment. This is, however, a common
phenomenon, manifest also in other structures,23–25 and prob-
ably implies a degree of interaction between Ce and both lone
pairs; indeed, in this very structure, the orientations of the
hypophosphite ligands also are not consistent with an sp3
hybridisation at oxygen atoms and direct alignment between
only one oxygen lone pair and Ce. The overall co-ordination
geometry (Fig. 3) around Ce conforms most closely to bicapped
trigonal prismatic (BTP), although polytopal analysis shape
parameters26 indicate that a degree of square antiprismatic
(SAP) character is also present (Table 4; DOD = dodecahedral)
which is manifest mainly in the elongation of the O(1) ؒ ؒ ؒ O(31)
distance so as to destroy the coplanarity of the sides of the
prism.
µ4(κ2, κ1, κ1, κ1)
As in [Ce(H2PO2)3(H2O)], the water H atoms were not defini-
tively located, but were clearly implied by interoxygen distances
and angles: as a result it is clear that the structural units
are bound together by means of the non-ligand water mol-
ecules, which bridge between hypophosphite O atoms as
hydrogen-bonding acids, in turn acting as hydrogen-bonding
bases to the ligand waters (Fig. 6). The result is a region of
the structure inhabited by water molecules associated by
hydrogen-bonding and extending along the screw axes. The
space group itself being chiral, this area resembles a ‘screw
thread’ of hydrogen bonds. Again, as in [Ce(H2PO2)3(H2O)], the
geometry at the ligand water is far from tetrahedral. The
O(2) ؒ ؒ ؒ O(1) ؒ ؒ ؒ O(2)xii angle is almost tetrahedral, but the
corresponding angles between Pr and O(2) or O(2)xii (these are
the O atoms which act as hydrogen-bonding bases to H(13) and
H(14) respectively) are obtuse; as in [Ce(H2PO2)3(H2O)], no
other close contacts are made with O(1) to imply a demand on
the remaining lone pair, the implication being, in view of the
angles at O(1), that it interacts somewhat with Pr. The geometry
at O(2) is more regular (Table 6). The mean hypophosphite P–O
The important bond distances and angles of the type B
complex [Pr(H2PO2)3] have been communicated elsewhere.17
Its overall co-ordination geometry conforms fairly closely to
square antiprismatic (Table 4).
The structure reported18 for [Er(H2PO2)3] and that deter-
mined by us for our dysprosium type C complex are different,
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Table 4 Polytopala analysis shape parameters for 8-co-ordinate lanthanide hypophosphite complexes
[Ce(H2PO2)3(H2O)]
[Pr(H2PO2)3]b
[Pr(H2PO2)(HPO3)(H2O)]ؒH2O
BTP
SAP
DOD
δ 1(57)3/Њ
δ 2(68)4/Њ
δ 2(58)3/Њ
δ 1(67)4/Њ
φ 7–1–2–8/Њ
φ 5–3–4–6/Њ
13.8
2.9
45.6
42.9
16.0
16.6
8.0
7.6
50.0
57.4
24.2
37.7
34.1
16.0
50.7
45.6
5.1
21.8
0.0
48.2
48.2
14.1
14.1
0.0
0.0
52.5
52.5
24.5
24.5
29.5
29.5
29.5
29.5
0.0
20.4
0.0
a Vertices are assigned, for the above three structures respectively, as follows: 1 [O(12)ii, O(11), O(21)], 2 [O(21), O(21), O(22)], 3 [O(32)iii, O(31),
O(23)xi], 4 [O(22)i, O(12)xv, O(1)], 5 [O(31), O(32)xix, O(11)], 6 [O(12)i, O(22)iv, O(12)ix], 7 [O(1), O(22), O(22)ix], 8 [O(11), O(12)xi, O(21)x]. b See ref. 17.
Table 5 Selected interatomic separations (Å) and angles (Њ) for
[Dy(H2PO2)3]
Dy(1)–O(11)
Dy(2)–O(12)
P(1)–O(12)
P(2)–O(21)
2.243(6)
2.246(6)
1.439(7)
1.486(5)
Dy(1)–O(21)
Dy(2)–O(31)
P(1)–O(11)
P(3)–O(31)
2.256(4)
2.259(5)
1.465(7)
1.489(5)
O(11)–Dy(1)–O(21)
O(21)vi–Dy(1)–O(21)vii
O(11)–Dy(1)–O(11)vi
O(12)–Dy(2)–O(31)
O(31)–Dy(2)–O(31)vii
O(12)i–Dy(2)–O(12)
O(12)–P(1)–O(11)
91.3(2)
89.5(2)
180.0
88.4(2)
90.1(2)
180.0
O(11)–Dy(1)–O(21)vi
O(21)–Dy(1)–O(21)vii
O(21)–Dy(1)–O(21)vi
O(12)i–Dy(2)–O(31)
O(31)i–Dy(2)–O(31)vii
O(31)–Dy(2)–O(31)i
O(21)–P(2)–O(21)viii
88.7(2)
90.5(2)
180.0
91.6(2)
89.9(2)
180.0
Fig. 6 The packing arrangement of [Pr(H2PO2)(HPO3)(H2O)]ؒH2O
seen22 along the unit cell a axis. Some bonds are omitted for clarity. The
symbol X denotes the approximate location of a screw axis consisting
the centre of a ‘screw thread’ of water molecules.
121.1(5)
117.0(4)
116.6(4)
O(31)–P(3)–O(31)viii
Fig. 7 The co-ordination around the Pr atom in [Pr(H2PO2)(HPO3)-
(H2O)]ؒH2O.
Fig. 5 A view22 of [Pr(H2PO2)(HPO3)(H2O)]ؒH2O. The Pr, P, O and H
atoms are depicted with decreasing arbitrary radii. Only H atoms, and
their hydrogen-bonds to surrounding atoms, are shown for purposes of
clarity.
radius sphere, at a metal’s centre, which is covered by conic
projections of the ligating atoms. Computationally this is done
by summing all of the individual atoms’ contributions but,
since overlap between the individual atoms’ projections is not
counted twice, an overlap correction is then applied. Values
calculated for the above complexes are presented in Table 7
along with those for some related complexes. The table is
arranged according to atomic weight of the metal, and the
effects of the lanthanide contraction are immediately evident in
the general increase in SAS values for the later lanthanides. The
lanthanum complex has the smallest value, and there is no over-
lap correction required. In the case of the isostructural type A
cerium complex a very small overlap correction is present,
which arises from a close approach of atoms O(12)i and O(12)ii
which reside in two µ3(κ1,κ1,κ1)-hypophosphite ligands. This
contact may be expected to become closer in the later lan-
thanide structures, and suggests that the structural change from
A to AЈ occurs when it reaches a limiting value. The SAS calcu-
lation for the type AЈ [Eu(H2PO2)3(H2O)] requires an overlap
correction, but this arises mainly from a contact between the
distance of 1.491[4] Å in this structure appears to be shorter
than the 1.513[2] Å seen in [Pr(H2PO2)3], but this is to be inter-
preted in the light of the fact that both the co-ordination mode
of the hypophosphites and the nature of the coligands is differ-
ent in both structures. In the phosphite ligand the P(2)–O(23)
distance is shorter than the distances to O(21) and O(22), which
situation has been observed before27,28 and is analogous to the
asymmetry of the µ3(κ1,κ1,κ1)-hypophosphite ligands, caused
by different ligating demands on the two O atoms. In [Pr(H2-
PO2)(HPO3)(H2O)]ؒH2O the co-ordination polyhedron is a
dodecahedron, perhaps rather distorted in the direction of SAP
by the loss of planarity of the O(11), O(23)xi, O(1), O(12)ix
trapezoid (Fig. 7).
The degree of steric crowding in the above complexes can be
compared by the use of steric angle sum (SAS) calculations,1
which express the fraction of surface area of a notional 1 Å
J. Chem. Soc., Dalton Trans., 1999, 2189–2196
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Table 6 Selected interatomic separation (Å) and angles (Њ) for [Pr(H2PO2)(HPO3)(H2O)]ؒH2O
Pr–O(11)
2.479(6)
2.570(5)
2.479(6)
2.395(5)
1.478(6)
1.540(6)
1.490(6)
2.827(9)
2.866(8)
Pr–O(21)
2.639(6)
2.475(7)
2.390(6)
2.367(6)
1.505(7)
1.547(6)
2.784(9)
2.794(8)
Pr–O(22)
Pr–O(1)
Pr–O(12)ix
Pr–O(21)x
Pr–O(22)ix
Pr–O(23)xi
P(1)–O(12)
P(2)–O(22)
O(1) ؒ ؒ ؒ O(2)
O(2) ؒ ؒ ؒ O(12)xiii
P(1)–O(11)
P(2)–O(21)
P(2)–O(23)
O(1) ؒ ؒ ؒ O(2)xii
O(2) ؒ ؒ ؒ O(11)xiv
O(23)xi–Pr–O(21)x
O(21)x–Pr–O(22)ix
O(21)x–Pr–O(1)
90.82(20)
171.07(21)
79.98(23)
132.99(21)
82.07(20)
77.29(23)
94.55(20)
147.52(21)
66.84(19)
123.70(22)
77.55(20)
122.41(15)
137.49(20)
77.99(22)
118.86(33)
113.05(34)
133.35(30)
101.48(26)
112.18(32)
O(23)xi–Pr–O(22)ix
O(23)xi–Pr–O(1)
80.27(20)
70.85(21)
96.31(23)
103.82(20)
68.29(21)
84.06(20)
143.93(23)
147.80(19)
121.53(13)
76.93(20)
135.61(19)
65.63(18)
71.17(19)
56.01(18)
114.37(32)
104.86(33)
123.82(27)
88.62(27)
114.58(29)
O(22)ix–Pr–O(1)
O(23)xi–Pr–O(12)ix
O(22)ix–Pr–O(12)ix
O(23)xi–Pr–O(11)
O(22)ix–Pr–O(11)
O(12)ix–Pr–O(11)
O(21)x–Pr–O(22)
O(1)–Pr–O(22)
O(21)x–Pr–O(12)ix
O(1)–Pr–O(12)ix
O(21)x–Pr–O(11)
O(1)–Pr–O(11)
O(23)xi–Pr–O(22)
O(22)ix–Pr–O(22)
O(12)ix–Pr–O(22)
O(23)xi–Pr–O(21)
O(22)ix–Pr–O(21)
O(12)ix–Pr–O(21)
O(22)–Pr–O(21)
O(11)–Pr–O(22)
O(21)x–Pr–O(21)
O(1)–Pr–O(21)
O(11)–Pr–O(21)
O(11)–P(1)–O(12)
O(23)–P(2)–O(22)
Pr–O(1) ؒ ؒ ؒ O(2)
O(2) ؒ ؒ ؒ O(1) ؒ ؒ ؒ O(2)xii
O(1) ؒ ؒ ؒ O(2) ؒ ؒ ؒ O(11)xiv
O(23)–P(2)–O(21)
O(21)–P(2)–O(22)
Pr–O(1) ؒ ؒ ؒ O(2)xii
O(1) ؒ ؒ ؒ O(2) ؒ ؒ ؒ O(12)xiii
O(11)xiv ؒ ؒ ؒ O(2) ؒ ؒ ؒ O(12)xiii
Table 7 Steric angle sums (SASs) for lanthanide hypophosphite
complexes
radii of P and O for these type A and B complexes both of
which undergo thermal disproportionation.
SAS
Conclusion
Complex
Uncorrected
Corrected
Whilst the hypophosphite–lanthanide system has not proven
uncomplicated in its behaviour, relatively simple analyses, such
as SAS calculations, appear to have afforded some fundamental
insights to the structural preferences and reactivities of lan-
thanide hypophosphite complexes. These studies have estab-
lished the classes of structures adopted by these materials and
the ranges over which each is formed in the lanthanide series.
The structure of type A has been verified by examination of a
new member, and that of type C established correctly for the
first time. No evidence for a 7-co-ordinate structural type,
between the former 8- and the latter 6-co-ordinate complexes,
has been found. A new class of mixed-ligand hypophosphite–
phosphite complex, containing interesting chiral channels of
water molecules, has been synthesized and characterised
structurally.
[La(H2PO2)3(H2O)]a
[Ce(H2PO2)3(H2O)]
[Pr(H2PO2)3]b
[Pr(H2PO2)(HPO3)H2O]ؒH2O
[Eu(H2PO2)3(H2O)]c
[Dy(H2PO2)3]
0.653
0.691
0.698
0.705
0.738
0.6503, 0.6486
0.653
0.691
0.698
0.699
0.733
0.6503, 0.6486
a See ref. 15. b See ref. 17. c See ref. 16.
µ3(κ1,κ1,κ1) and the water ligands, rather than between like
ligands.
The type B [Pr(H2PO2)3] contains no strong contacts such as
would necessitate an overlap correction, despite possessing a
larger SAS value than [Ce(H2PO2)3(H2O)] and being presum-
ably more sterically crowded. The distribution of ligand
atoms17 around the metal ion in the type B materials is evi-
dently quite efficient, which is in accordance with the observed
SAP co-ordination geometry. This might explain why type B
forms by dehydration of type A, even in the presence of
water.
The complex [Dy(H2PO2)3] contains two crystallographically
inequivalent dysprosium ions, and thus has two ascribable SAS
values, the comparatively small magnitudes of which reflect the
fact that this structure is a 6-co-ordinate one. This allows the
ligands to move well away from one another, which is manifest
in the lack of any overlap correction and the lower density of
[Dy(H2PO2)3] compared to17 [Pr(H2PO2)3]. Furthermore, it
might also afford type C complexes their resistance to dispro-
portionation of hypophosphite, which occurs readily in types
A, AЈ and B at elevated temperatures. Since one of the products
of this process is phosphine, interphosphorus oxygen transfer is
a requisite, but there are no P ؒ ؒ ؒ O approaches in [Dy(H2PO2)3]
closer than 3.508(6) Å. This can be compared with values of
3.126(5) Å in [Ce(H2PO2)3(H2O)] and 2.918(4) Å in [Pr(H2-
PO2)3], both of which fall well inside the sum of van der Waals
Experimental
Instrumentation
JEOL JNM-FX270 and FX90Q (NMR), ATI Mattson Genesis
single beam (FTIR) Philips 1050 Cu-Kα (powder diffraction)
and Netzch STA 409 EP (TGA) instruments were used for
routine analysis.
Analysis
Oxidisable phosphorus was determined by treatment of
samples with an excess of bromine (generated quantitatively
from bromate and bromide). The sample was stirred with brom-
ine for 3 h in a stoppered flask, cooled in ice and an excess of
potassium iodide added. The iodine liberated was titrated with
standard sodium thiosulfate solution. For parity of reporting
for pure hypophosphite and mixed-ligand systems, the results
are expressed in terms of calculated molar mass. Percentage
phosphorus values were determined by DCP (direct current
plasma) atomic absorption spectroscopy.
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J. Chem. Soc., Dalton Trans., 1999, 2189–2196

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Syntheses
mon refined P–H distance (1.245(15) Å) was used for the
refinement. The largest hole and peak in the final Fourier-
difference map were Ϫ0.86 and 0.74 e ÅϪ3 (ca. 1.2 Å from Dy(1)
and O(11)).
[Ln(H2PO2)3(H2O)n] (types A and AЈ, n ؍ 1; C, n ؍ 0), gen-
eral procedure. Sodium hypophosphite (ca. 1.00 g) was dis-
solved in pH 1.4 (KCl–HCl) buffer solution (10 ml). Hydrated
lanthanide chloride (ca. 1.20 g) in the buffer solution (10
ml) was added. The mixture was allowed to stand in air at
room temperature for one hour. A solid precipitated from
solution where Ln = La–Eu, Dy–Lu. For Ln = Gd or Tb
addition of a seed crystal resulted in rapid precipitation from
the supersaturated solution. The resulting solids, bearing the
characteristic colour of the Ln3ϩ ions, were collected by suction
filtration, washed with water, and dried at the pump. Yields 61–
89%. Representative analyses: [Ce(H2PO2)3(H2O)], found P
26.1, H8CeO7P3 requires 26.3%; [Dy(H2PO2)3], found P 25.6.
H6DyO6P3 requires 26.0%; [Sm(H2PO2)3(H2O)], M found 366,
theoretical 363; [Eu(H2PO2)3(H2O)], M found 362, theoretical
365.
[Pr(H2PO2)(HPO3)(H2O)]ؒH2O. The structure was solved by
Patterson methods.32
A common refined P–H distance
(1.266(26) Å) was used, which was considered a reasonable step
in the light of the similarity of the IR stretching frequencies.
Some systematic error in the data necessitated that phosphite O
atoms be weakly restrained to be isotropic to prevent ‘non-
positive definite’ collapse. Water H atoms were restrained to sit
30% along the length of their putative hydrogen-bonding vec-
tors and, as a result, the refinement was unstable and required
damping. Final error limits were estimated using undamped
cycles of refinement. The largest hole and peak in the final
Fourier-difference map were Ϫ1.29 and 3.99 e ÅϪ3 (at Pr).
CCDC reference number 186/1450.
[Ln(H2PO2)3] (type B). Stoichiometric amounts of sodium
hypophosphite and the lanthanide chloride were dissolved sep-
arately in the minimum amount of water. On mixing a precipi-
tate was formed immediately, and the reaction mixture heated
under reflux under nitrogen for several hours. Needle-like crys-
tals formed of the colour of the corresponding Ln3ϩ ion, which
were collected by suction filtration, washed with water, and
dried at the pump. Yields 51–69%. [Pr(H2PO2)3)], M found 336,
theoretical 336.
Acknowledgements
The authors gratefully acknowledge the use of the EPSRC’s
X-Ray Crystallography Service, and Chemical Database
Service at Daresbury. Thanks are due to Stephanie Barnett for
simulations of powder patterns, and to all in the Divison of
Ceramics for use of apparatus.
References
[Tb(H2PO2)3] (type C) from [Tb(H2PO2)3(H2O)] (type AЈ). A
small quantity of [Tb(H2PO2)3(H2O)] was stirred under reflux
with excess of trimethyl orthoformate in a nitrogen atmosphere
for 2 h. The resulting material was filtered in a dry-box, washed
with dry thf and allowed to dry.
1 X.-Z. Zhang, A.-L. Guo, Y.-T. Xu, X.-F. Li and P.-N. Sun,
Polyhedron, 1987, 6, 1041.
2 F. A. Hart, in Comprehensive Coordination Chemistry, eds. G.
Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon Press,
Oxford, 1987, vol. 3.
3 M. D. Marcos, R. Ibanez, P. Amoros and A. LeBail, Acta
Crystallogr., Sect. C, 1991, 47, 1152.
[Ln(H2PO2)(HPO3)(H2O)n]ؒH2O (type 1, n ؍ 1; 2, n ؍ 0).
Air oxidation of the lanthanide hypophosphite complexes,
monitored by IR spectroscopy, was achieved by in situ air oxid-
ation on heating the lanthanide chloride with sodium hypo-
phosphite in aqueous solution on a steam-bath for several
hours. Needle like crystals were formed for the praseodymium
and neodymium complex, whilst for the later lanthanides the
complexes were obtained as fine powders. The products were
filtered off, washed with water and dried in air at the pump.
Representative analyses: [Nd(H2PO2)(HPO3)(H2O)]ؒH2O, M
found 301, theoretical 325; [Tm(H2PO2)(HPO3)], M found 316,
theoretical 314; [Lu(H2PO2)(HPO3)], M found 325, theoretical
320.
4 T. J. R. Weakley, Acta Crystallogr., Sect. B, 1979, 35, 42.
5 M. D. Marcos, P. Amoros, F. Sapina, A. Beltran-Porter,
R. Martinez-Manez and J. P. Attfield, Inorg. Chem., 1993, 32, 5044.
6 T. Matsuzaki and Y. Itaka, Acta Crystallogr., Sect. B, 1969, 25, 1932.
7 B. O. Loopstra, Int. Estab. Nucl. Energy Res. Publ., 1958, 15, 64.
8 A. R. Pedrazuela, S. Garcia-Blanco and L. Rivoir, An. Soc. Esp. Fis.
Quim., 1953, 49, 255.
9 T. Akimoto, Ph.D. Thesis, University of Tokyo, 1965.
10 T. J. R. Weakley, J. Chem. Soc. Pak., 1983, 5, 279.
11 T. J. R. Weakley and W. W. L. Watt, Acta Crystallogr., Sect. B, 1979,
35, 3023.
12 P. A. Tanner, T. Sze, T. C. Mak and W. Yip, J. Cryst. Spectrosc. Res.,
1992, 22, 25.
13 P. A. Tanner, S.-T Hung, T. C. W. Mak and W. Ru-Ji, Polyhedron,
1992, 11, 817.
14 W. H. Zachariasen and R. C. L. Mooney, J. Chem. Phys., 1934, 2, 34.
15 V. M. Ionov, L. A. Aslanov, V. B. Rybakov and M. A. Porai-Koshits,
Kristallografiya, 1973, 18, 403.
16 V. M. Ionov, L. A. Aslanov, M. A. Porai-Koshits and V. B. Rybakov,
Kristallografiya, 1973, 18, 405.
17 J. A. Seddon, A. R. W. Jackson, R. A. Kresinski and A. W. G. Platt,
Polyhedron, 1996, 15, 1899.
18 L. A. Aslanov, V. M. Ionov, M. A. Porai-Koshits, V. G. Lebedev,
B. N. Kulikovskii, O. N. Gilyarov and T. L. Novoderezhkina, Izv.
Acad. Nauk. SSSR, Neorg. Mater., 1975, 11, 96.
19 J. Laane and J. R. Ohlsen, Prog. Inorg. Chem., 1980, 27, 465.
20 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th edn., Wiley, New York, 1986.
21 J. W. Mellor, Comprehensive Treatise on Inorganic and Theoretical
Chemistry, Longmans, New York, London, 1971, vol. 8, suppl. 3.
22 E. K. Davies, SNOOPI, Chemical Crystallography Laboratory,
University of Oxford, 1982.
23 The United Kingdom Chemical Database Service, D. A Fletcher,
R. F. McMeeking and D. Parkin, J. Chem. Inf. Comput. Sci., 1996,
36, 746.
24 D. L. Faithfull, J. M. Harrowfield, M. I. Ogden, B. W. Skelton,
K. Third and A. H. White, Aust. J. Chem., 1992, 45, 583.
25 M. C. Favas, D. L. Kepert, B. W. Skelton and A. H. White, J. Chem.
Soc., Dalton Trans., 1980, 454.
Crystallography
Diffraction data were collected, using Mo-Kα radiation,
according to previously published procedures.29 Non-H atoms
were modelled30 anisotropically, and H atoms isotropically,
these being placed in theoretical positions. Following con-
vergence of full-matrix least-squares refinement30 using merged
data, these were corrected31 for absorption effects and refine-
ment resumed to convergence.
[Ce(H2PO2)3(H2O)]. The structure was solved by isomorph-
ous replacement.15 A common refined P–H distance (1.309(14)
Å) was used, and water H atoms were restrained to sit 30%
along the length of their putative hydrogen-bonding vectors
and, as a result, the refinement was unstable and required slight
damping. Final error limits were estimated using undamped
cycles of refinement. The largest hole and peak in the final
Fourier-difference map were Ϫ1.08 and 1.40 e ÅϪ3 (at Ce).
[Dy(H2PO2)3]. Data were collected as for triclinic symmetry,
and transformed to the above system. The structure was solved
by trial placement of heavy atoms at special positions. A com-
26 M. A. Porai-Koshits and L. A. Aslanov, J. Struct. Chem., 1978, 13,
244.
J. Chem. Soc., Dalton Trans., 1999, 2189–2196
2195

View Article Online
27 Y.-P. Zhang, H. Hu and A. Clearfield, Inorg. Chim. Acta, 1992, 193,
35.
28 J.-D. Foulon, N. Tijani, J. Durand, M. Rafiq and L. Cot, Acta
Crystallogr., Sect. C, 1993, 49, 1.
31 N. Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983, 39, 158.
32 G. M. Sheldrick, SHELXS 86, Program for the Solution of Crystal
Structures, University of Göttingen, 1986.
29 J. A. Darr, S. R. Drake, M. B. Hursthouse and K. M. A. Malik,
Inorg. Chem., 1993, 32, 5704.
30 G. M. Sheldrick, SHELXL 93, Program for Crystal Structure
Refinement, University of Göttingen, 1993.
Paper 8/07727D
2196
J. Chem. Soc., Dalton Trans., 1999, 2189–2196