Physicochemical studies on some metal chelates of cyclodiphosphazane and thiourea monophosphazene derivatives

Article abstract of DOI:10.1080/10426500701501417

The interaction of Cr(III), Mn(II), and Fe(III) ions with cyclodiphosphazane and thiourea monophosphazene derivatives was followed potentiometrically with the aim of determination of stability constants of (metal : ligand) 1:1 and 1:2 binary, and 1:1:1 ternary chelates at 25C and = 0.1 mole dm- 3 NaNO3 in 10% (v/v) H2O-EtOH mixture. The stability of the complexes of both ligands was found to be Fe (III) Mn(II) Cr(III). The concentration distribution of the species formed in solution was evaluated. Binary and ternary solid complexes were prepared with the purpose of structural characterization using elemental analyses, molar conductance, IR, 1HNMR and electronic spectra and thermogravimetric analysis. The data show that the ligands behave as neutral bidentate ligands, forming neutral chelates of monomeric nature.

Full text of DOI:10.1080/10426500701501417

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Physicochemical Studies
on Some Metal Chelates of
Cyclodiphosphazane and
Thiourea Monophosphazene
Derivatives
A. A. M. Gad a , G. M. H. Taha a , A. E. Arifien a & M.
Sh. Zoromba a
a Chemistry Department, Aswan, Faculty of
Science , South Valley University , Egypt
Published online: 23 Aug 2007.
To cite this article: A. A. M. Gad , G. M. H. Taha , A. E. Arifien & M. Sh. Zoromba
(2008) Physicochemical Studies on Some Metal Chelates of Cyclodiphosphazane and
Thiourea Monophosphazene Derivatives, Phosphorus, Sulfur, and Silicon and the
Related Elements, 182:10, 2425-2438, DOI: 10.1080/10426500701501417
To link to this article: http://dx.doi.org/10.1080/10426500701501417
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Phosphorus, Sulfur, and Silicon, 182:2425–2438, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701501417
Physicochemical Studies on Some Metal Chelates of
Cyclodiphosphazane and Thiourea Monophosphazene
Derivatives
A. A. M. Gad
G. M. H. Taha
A. E. Arifien
M. Sh. Zoromba
Chemistry Department, Aswan, Faculty of Science, South Valley
University, Egypt
The interaction of Cr(III), Mn(II), and Fe(III) ions with cyclodiphosphazane and
thiourea monophosphazene derivatives was followed potentiometrically with the
aim of determination of stability constants of (metal : ligand) 1:1 and 1:2 binary, and
1:1:1 ternary chelates at 25◦C and μ = 0.1 mole dm−3 NaNO3 in 10% (v/v) H O-
2
EtOH mixture. The stability of the complexes of both ligands was found to be Fe (III)
> Mn(II) < Cr(III). The concentration distribution of the species formed in solution
was evaluated. Binary and ternary solid complexes were prepared with the purpose
of structural characterization using elemental analyses, molar conductance, IR,
1HNMR and electronic spectra and thermogravimetric analysis. The data show
that the ligands behave as neutral bidentate ligands, forming neutral chelates of
monomeric nature.
Keywords Cyclodiphosphazane; metal chelates; potentiometry; thioureamonophosp-
hazene
INTRODUCTION
Phosphorus-containing compounds are widely used in phosphate laser
glasses, binders and coatings in refractory ceramics, in the intermediate
products of the chemical industrial synthesis, in agricultural fertilizers,
also, phosphoric acid and phosphate salts can be used to retard corrosion
of metals.1−5 The ester compounds of phosphoric acid play a central role
in cellular metabolism.
The literature survey reveals that this class of compounds had con-
siderable attention during the last decade, due to their remarkable
Received February 25, 2007; accepted May 17, 2007.
Address correspondence to A. Gad, Chemistry Department, Faculty of Science, South
Valley University, Aswan, Egypt. E-mail: a gad53@yahoo.com
2425

2426
A. A. M. Gad et al.
catalytic, medical, polymers, and semiconductor applications6. The
great use of the compounds is due to the different chemical behavior of
N- and P-sites7−11. Nitrogen as a hard donor, is capable of stabilizing
metals in high oxidation states; the chelate effect may allow unusual
coordination numbers and geometries. Phosphorus and sulfur as soft
bases are best suited to stabilize metals in low to medium oxidation
states. Thus, in view of increasing interest and in continuation of our
work on the structure of metal chelates of these compounds,12,13 we
report, herein, the interactions of linear thiourea monophosphazene
(TMPZ, L1) and cyclodiphosphazane (CDPZ, L2) derivatives (seen be-
low), with Cr(III), Mn(II), and Fe(III) both in solution and in solid phase.
Our goal for this article is to throw more light on the effect of stere-
ochemistry of the ligands on their coordination behavior. Our study
involves the determination of stability constants of the chelates formed
in solution using pH-metric technique (Scheme 1).
SCHEME 1
EXPERIMENTAL
All chemicals used were of A.R. grade and used without further purifica-
tion. Thioureamonophosphazene (TMPZ = L1) and cyclodiphosphazane
(CDPZ = L2), were prepared according to the published procedures,14,15
and their structures were confirmed with IR and elemental analy-
sis which were carried out at the Microanalytical Center in Cairo
University.
Stock solutions of the ligands were prepared by dissolving the req-
uisite amounts in 10% (v/v) H2O-EtOH. The stock solutions of chloride
salts of Cr(III), Mn(II), and Fe(III) ions were prepared in doubly dis-
tilled water and standardized as given by West16. Carbonate-free solu-
tion of NaOH (0.1 mol·dm−3) was prepared and standardized against

Physicochemical and Thiourea Monophosphazene Derivatives
2427
potassium hydrogen phthalate solution. 1.0 mol·dm−3 NaNO3 stock so-
lution was prepared immediately prior to use.
pH-metric titrations were carried out at 25◦C 0.1◦C in a doubly walled
glass cell and the temperature was maintained constant by a thermo-
stat. The pH values were determined using a MV-89 digital pH-meter
with an accuracy of 0.005 units. The pH mode was calibrated before
and after each titration using standard buffers at pH 4.0 and 7.0. The
following mixtures were titrated against CO2-free 5 × 10−2 mol·dm−3
NaOH: HNO3 1 × 10−2 mol·dm−3(a), a + (L1) or/and (L2) 5 × 10−3
mol·dm−3 (b), b + metal ion 1 × 10−3 mol·dm−3(c). The total volume
was adjusted to 50 cm3 by adding double–distilled H2O in each titra-
tion and the ionic strength was adjusted to 0.1 mol·dm−3 using NaNO3.
A magnetic stirrer was performed during all titrations and oxygen-free
nitrogen gas was bubbled through the solution before and during the
titrations. Multiple titrations were carried out for each system. The
pH-meter readings in 10% (v/v) EtOH-H2O were converted to [H+] ac-
cording to the relation of Van Uitert and Hass.17
Binary and ternary solid complexes were prepared by mixing,
with heating, the requisite amounts, 0.1 mmol (1.96, 2.66, 2.69 g for
MnCl2.4H2O, CrCl3.6H2O, and FeCl3.6H2O, respectively) of aqueous
metal salts with the appropriate amounts of the ligands (4.99 and 6.05
g for L1 and L2, respectively, duplicating the amount of L1 to prepare
the 1:2 Fe3+ binary chelate) in ethanol, refluxing for about 5 h. After
leaving overnight the complexes were then filtered and dried in vacuo
over P4O10. The solid complexes are soluble in polar organic solvents
and in alcohols. The electronic spectra were performed using Shimadzu
UV-160-Visible Recording Spectrophotometer using 1 cm width quartz
cell. Molar conductance values of ethanol solutions of the synthesized
complexes were measured at 25◦C using a model 31 YSI conductivity
bridge with a conductivity cell constant = 0.10 m−1. The other meth-
ods of study were carried out in the Micro analytical Center in Cairo
University.
Results and Discussion
It is worth mentioning that the ligands studied do not undergo hy-
drolysis under the experimental conditions as indicated from the rapid
attainment of equilibrium and no change in pH during the time of titra-
tion, ruling out any hydrolysis and thus the formation of hydroxo species
under the experimental conditions. Figure 1(a) shows representative
sets of experimental titration curves obtained according to the sequence
described in the experimental section for the different systems studied.

2428
A. A. M. Gad et al.
FIGURE 1 Titration curves of: (a) 0.01 mol·dm−3 HNO3, (b) soln. (a) + 0.05
mol·dm−3 ligand (L1); (c) mixture (b) + 0.001 mol·dm−3 Cr(III); (d) soln. (a)
+0.05 mol·dm−3 ligand (L2); (e) mix. (d) + 0.001 mol·dm−3 Cr(III); (f) mixture
(e) + 0.05 mol·dm−3 ligand (L1); t = 25◦C, μ = 0.1 mol·dm−3 NaNO3.
The titration curves of the free ligands Figure 1(b) exhibit three (L1)
or two (L2) inflections at a ([OH−]/[L]) value of 1, 2, and 3 respectively.
The presence of an inflection at a ratio equals 1, which represents the
protonation steps of the ligands. The cationic forms are dominant at pH
3.5–6.3, the mono- and bi- anionic forms are dominant species at pH
FIGURE 2 pH-species distribution profile for L1 species.

Physicochemical and Thiourea Monophosphazene Derivatives
2429
TABLE I pk Values of Thiourea Monophsphazene (L1) and
Cyclodiphosphazan (L2)
Ligand
Pk1
Pk2
Pk3
Thiourea monophosphazene (L1)
Cyclodiphosphazane (L2)
3.61 (3.50)∗
4.95 (4.80)
4.95 (5.00)
8.93 (8.90)
9.15 (9.15)
—
∗ref. (20).
6.3–8.5. These equilibria can be represented as follows:
(1)
The values of n¯ A (average number of protons attached per ligand) has
been determined making use of Irving and Rossotti18,19 method using
curves of Figure 1(a) and (b). The proton-ligand dissociation constants
(pK’s) were calculated from the n¯A- pH graphs, where the pK is the pH
value corresponding to n¯A = 0.5, 1.5, and 2.5. The obtained pK values
are given in Table (1). These pK values are closely in line with those
obtained spectrophotometrically.20
On the addition of the metal ions to the solution of the free ligand,
it is observed that the curve of Figure 1(c) is well separated from curve
(b). This is an indication of the liberation of protons during complex-
formation, and thus the complex [curve (c)] is located under curve (b),
i.e., at lower pH values. The titration curves were used to construct the
formation curves as a relation between n (average number of ligand
molecules attached per metal ion) and pL (free ligand exponent). From
these curves, the values of the stability constants were determined and
listed in Table II. The titration curves of complexes show inflections at
a = 2 in case of Fe(III) complexes, and inflections at a = 1 and 2 for the
TABLE II Stability Constants for Binary
Systems in 10% EtOH/H2O, at 25◦C
Complex
Cr3+-L1
Ratio
Log K
1:1
1:2
1:1
1:2
1:2
1:1
1:2
1:1
1:2
1:1
1:2
4.25
4.13
4.23
4.10
4.27
4.15
4.08
4.14
4.00
4.17
4.10
Mn2+-L1
Fe3+-L1
Cr3+-L2
Mn2+-L2
Fe3+-L2

2430
A. A. M. Gad et al.
Cr(III) and Mn(II) complexes (a = number of moles of base added per
mole of metal ion). This indicates that the complexation between the
ligand (L) and the metal ion lead to the formation of 1:1 and 1:2 (M:L)
complex species for Cr(III) and Mn(II), while only 1:2 can be detected
for Fe(III).
As shown in Figure 1(a), the M-CDPZ systems behave similarly to
M-TMPZ, except that the L2-metal complex [curve (d)] is lower in po-
sition compared to that of L1 confirming that the stability of L2-metal
complexes is less than that of L1(see Table II). Accordingly, we can sug-
gest that the difference in stability of the two types of complexes may
be explained in the light of the following: 1) thiourea monophosphazene
(L1) forms chelates of six-member ring, which is more stable than the
five-member (open structure) chelates formed by cyclodiphosphazane
(L2); 2) complexes of L2 contain the four-member cyclodiphosphazane
ring which may cause steric hindrance that result in decreasing the
stability of the formed complexes; and 3) L2 has backbone rigidity im-
posed by the ring system, which is most likely an origin of its weak
donor ability.
As can be seen from Table II, the stability constants of the binary
complexes decrease generally in the order Fe(III) > Cr(III) > Mn(II),
which is in agreement with Irving and Williams order.21
From the titration curves of ternary systems studied (Figure 1), one
can observe that the ternary system curves (f) are well separated from
binary system curves [(c), (e)] at pH > 3.5 for Cr(III)-L1 binary and at
pH > 6 for Cr(III)-L1–L2ternary systems. This behavior reveals that in
these pH ranges, coordination of secondary ligand with Cr(III) starts.
The overall formation constants (KMML L ) of ternary complexes can be
1
2
calculated considering that complexation takes place in simultaneous
manner (Eq. 2) or stepwise manner (Eq. 3) according to the following
equilibria:
(2)
(3)
Practically, the binary complex, ML is considered to be completely
formed in the lower buffer region followed by the addition of the sec-
ond ligand in the upper buffer region according to equilibrium (Eq. 2).

Physicochemical and Thiourea Monophosphazene Derivatives
2431
Thus, the overall stability constant βMML L may be represented by the
following equation:
1
2
ML1
ML1L2
βMML L
=
= K
− KMML
(4)
1
2
1
The horizontal distance between curves (c) and (f) (Figure 1a) can be
used to calculate n´mix (average number of secondary ligand molecules
associated with one [ML1] ion). From the values of n´mix, the secondary
ligand exponent, pLmix, the formation constants log KML are obtained
1
from the n´mix − pLmix plots, and the average values aMreL1rLe2ported in Ta-
ble III. The obtained data are treated using least square method22, the
standard deviations are found in the range of 0.2–0.08. The consistency
of the resulting parameters and estimated errors associated with mea-
surements are tested as previously reported.23
The relative stability of ternary metal complex (ML1L2) as compared
to that of the corresponding binary complex (ML1), has been quanti-
tatively expressed in terms of the parameter ꢀ log K, the difference
between the stabilities of binary and ternary chelates, as calculated
according to Equation (5):
ML1
ML1L2
ꢀlogK = logK
− log KMML
(5)
2
ꢀlog K values (Table III) were found to be slightly positive in accordance
with statistical consideration.24 These values suggest that the extent of
stabilization due to metal to ligand dπ - pπ back bonding causes metal
ion to be more positive (more electronegative).25 This permits stronger
interaction with the secondary ligand, hence there is no effect for the
steric and electrostatic factors, which result in lowering the stability
of ternary complexes as compared with binary complexes. However, in
these ternary systems the magnitude of ꢀlog K values are not very
large, indicating that ternary complexes may exist in both stacked and
open forms in equilibrium with each other in solution.
TABLE III Stability Constants and Δlog K Values for
1:1:1 Ternary Complexes in 10% EtOH/H2 O at 25◦C
ML
ML
1
1
Complex of
Log KMML
Log K
ꢀ log K
L
1
2
Cr3+-L1
4.25
4.15
4.23
4.14
—
6.16
1.91
2.01
1.6
1.69
—
Cr3+ - L2
Mn2+-L1
Mn2+ - L2
Fe3+-L1
5.83
6.46
Fe3+ - L2
4.17
2.29

2432
A. A. M. Gad et al.
FIGURE 3 pH-species distribution profile for the Binary and ternary M:L1:L2
systems.
Analysis of the species distribution plot of the free ligands (Fig-
ure 2), clearly indicate that at pH < 2 the ligand molecules become
fully protonated [α(H3O+) ≈78%]. At pH 2–3, both the protonated and
neutral forms coexist in equilibrium. The percentage of deprotonated
species HL− reaches to its maximum at pH ∼ 5. On further increase in
the pH values the HL− species decrease and L2− species increase at the
same time. At higher pH, the L2− species predominate, and the HL−
species have a very small extent.
A comparison of the species distribution curves for binary and
ternary systems is shown in Figure 3 for Cr(III)- H2TMPZ and Cr(III)-
CDPZ complexes (as representatives). Similar trends were obtained for
the other systems. The distribution profiles for these complexes show
that at lower pH values < 1.5, almost all Cr(III) ions are present as
free ions (98%). In the pH range 2–4.5, the predominant change is the
conversion of Cr(III) into Cr(III)- H2TMPZ complex with a maximum
degree of formation (30%). At a pH ∼ 2.5–4.0 the Cr(III) has completely
disappeared and Cr(III)- (L1)2 is present to an extent of (98%). As shown
in Figure 1(c), the distribution curves of Cr(III)-L1-L2 ternary systems
at pH > 8 are greatly different than those of binary system. The values
of maximum concentrations for 1:1:1 species were 62.5%, 34%, and 15%
for Fe(III), Cr(III), and Mn(II), respectively. At this range, the concen-
tration values of binary systems are of lower values.
The analytical data of the isolated solid complexes (Table IV), show
that the ligands act as neutral bidentate which form monomer chelates
of stoichiometric ratios 1:1, 1:2 binary, and 1:1:1 ternary complexes.

Physicochemical and Thiourea Monophosphazene Derivatives
2433
TABLE IV Elemental Analysis, Color and Molar Conductivity Values
(in mhos·Cm2/mole), of Binary and Ternary Solid Complexes
% Found (calculated)
Complex
C
H
N
S
M.wt.
Color
ꢁ
L1–Complexes (binary)
[Cr(C28H30N5PS)Cl3(H2O)]
Cr-L1(1:1)
[Mn(C28H30N5PS)Cl2].2H2O
Mn-L1 (1:1)
50,02 4.48 10.46 4.78 676.10 Blue
(49.75) (4.77) (10.36) (4.70)
51.13 4.83 10.92 5.01 661.50 Black
(50.84) (5.18) (10.58) (4.84)
57.85 2.17 12.17 5.50 1179.5 Yellow
(57.03) (2.73) (11.87) (5.42)
5.42
4.57
72.0
[Fe(C28H30N5PS)2Cl2]Cl..H2
Fe-L1 (1:2)
O
L2—Complexes (binary)
[Cr(C35H37N5P2O)Cl3(H2O)]
Cr-L2 (1:1)
[Mn(C35H37N5P2O)Cl2].2H2O 55.29 5.17
Mn-L2 (1:1)
54.19 5.16
(53.80) (5.02) (8.95)
9.23
(54.80) (5.38) (9.12)
53.90 5.13 9.15
(53.48) (5.00) (8.91)
9.19
—
—
—
782.16 Black
767.60 Black
786.01 Black
5.40
4.37
6.53
[Fe(C35H37N5P2O)Cl3(H2O)]
Fe-L2 (1:1)
L1, L2—Complexes (ternary)
[CrL1L2Cl2]Cl
60.47 5.53 11.47 2.62 1263.7 Greenish 68.4
(59.88) (5.34) (11.08) (2.53) blue
62.03 5.77 11.61 2.65 1231.2 Fair
[Mn L1L2Cl2]
4.56
64.5
(61.46) (5.49) (11.38) (2.60)
brown
[Fe L1L2Cl2]Cl.H2O
58.96 5.67 11.11 2.53 1285.6 Black
(58.86) (5.41) (10.90) (2.49)
The solubility of these complexes in common polar solvents makes one
excludes any dimeric or extended polymeric structures.
The molar conductivity values in DMF solution (1 × 10−3 mol·dm−3
)
are in the range of 4.37 – 6.53 ohm−1·cm2·mol−1, for binary (1:1) and
ternary Mn(II) complexes, revealing that they are non electrolytes.
While in case of ternary Cr(III) and Fe(III), as well as binary Fe(III) – L1
(1:2) complexes they are of higher values ( 64.5–72.0 ohm−1·cm2·mol−1),
which means the formation of 1:1 electrolytes. The small values reveal
the non- ionic neutral nature, whereas the higher molar conductivity
values may be due to partial displacement of chloro groups by DMF in
coordination sphere of the complex.26
IR spectra of binary and ternary complexes are recorded in Tables
V–VII and compared with those of the free ligands. The tables show
strong, mostly broad, bands in the region 3460–2922 cm−1, indicating
the combination of vOH stretching vibration (due to water coordinated
or hydrated in the complex molecules) and vNH present in the ligands.
The IR spectra also exhibited a doublet band at 730–782 cm−1 as-
signed to the cyclic vP-N mode of vibration. The other frequency located

2434
A. A. M. Gad et al.
TABLE V The Most Important Infrared Bands (cm−1) of Ligand L1
and Its Binary Complexes
Assignment of
L1
Fe(III)
Mn(II)
Cr(III)
IR bands
3421 br
3419 br
3450 br
v(OH) of
coordinated H2O
v(NH) stretching
δ(NH) deformation
3177, 3022 s 3187, 3065 m 3176, 3021 s
1691s,1602w 1691s, 1608 m 1691s, 1604 m
3179, 3023 m
1691 s, 1606 m
1493s,1455m 1493, 1451 m 1492, 1455 m 1494, 1455, 1423 m v(C S)
1162 m
1262 s
1165 m
1265 m
764 m
1166 m
1266s
763 m
1162 m
1263s
763 m
v(P N)
v of rocking mode
of coordinated
H2O
516 m,465 m 513 m, 478 m
250 m 287 m
510 m, 467 m
285 m
v(M N)
v(M Cl) orv(M S)
TABLE VI The Most Important Infrared Bands (cm−1) of Ligand L2
and Its Binary Complexes
L2
Fe(III)
Mn(II)
Cr(III)
Assignment
3419 br
2962, 2927 w
—
2362 m
1647, 1513 s
1179 s
1262 s
782 m
545 m, 450 m
307 m
3397 s
2980,2922 s
2583 s
2363 m
1578, 1492 s
1132 s
1277 s
777 m
521 m, 432 m
305 m
3393 br
2923 s
2588 s
v(OH)
v(NH)
exocyclic
v(P-NH)
δ(NH)
2922 s
2586 s
2362 m
1543 s
1181 s
1261 s
730 m
2363 m
1543, 1492 s
1177,1102 s
1277 s
768 m
578 m, 454 m
310 m
v(P O)
v(P N)
cyclic v(P N)
v(M N) or v(M O)
v(M Cl)
TABLE VII THe Most Important Infrared Bands (cm−1) of the
Ternary Complexes
Fe(III)
Mn(II)
Cr(III)
Assignments
3419 s
3187, 3022 m
1661 s
3420 br
3176-3023 m
1691s
3460 br
3179, 3020 m
1692 s
v(OH) orv(NH)
δ(NH)
1605 s
1603 s
1607 s
1450 m
1261 m
1173 s
1455 m
1238 m
1175 s
1455 m
1240 m
1176 s
v(C S)
v(P N)
v(P O)
755 m
782 m
768 m
v(P NH)
508 m, 475 m
290 w
513 m, 470 m
288 w
510 m, 470 m
289 w
v(M O) orv(M N)
v(M S) or v(M Cl)

Physicochemical and Thiourea Monophosphazene Derivatives
2435
at 1238–1277 cm−1 region is attributed to exocyclic vP N stretching.
The band observed at 2588–2362 cm−1 is characteristic of exocyclic
vP-NH ,and that at 1181–1102 cm−1 is due to vP=O mode of CPDZ
ligand. The new bands observed in the IR spectra of the complexes at
432–578 cm−1 region can be tentatively assigned to vM-L originated
from vM-O, coupling with vM-N.27 The band that is assigned to δN-H,
and appearing at about 1492–1692 cm−1 is unusually broad in the com-
plexes compared to that of the free ligand. The spectra of the complexes
also show a band in the 250–310 cm−1 range that can be assigned to the
trans-coordinated chloride atom (M-Cl). In TMPZ complexes, the vC=S
band, located at 1494–1423 cm−1 is slightly positively shifted. This is
an indication of the involvement of the S-atom in coordination. The
M-S stretching vibrations are expected to be found in the region 290–
250 cm−1. The appearance of this band in the present study confirms
coordination through the sulfur atom of TMPZ ligand.28−30
1
The H NMR spectral data agree well with the IR results in that,
the NH signals (four for L1 and two for L2) occur as high field signals
located at 5.2 and 4.8 ppm. The coordinated H2O molecules associated
to all binary complexes are evidenced by signals at about 4.0 ppm.31
Thermogravimetric analyses of the prepared binary and ternary
complexes have been carried out to confirm the suggested molecular
formulae of the prepared complexes. The obtained data reveal that the
binary Fe-L1(1:2) and ternary Fe-L1-L2 show weight losses in the tem-
perature range 110–120◦C due to loss of a molecule of hydrated H2O
from each type. The percentages of weight losses are 1.51 and 1.45%,
and the calculated values are 1.56 and 1.49%, respectively. Also, binary
Mn(II) complexes lose two hydrated water of crystallization.
The TG thermograms show also degradation for some chelates oc-
curring at about 165–215◦C with percentage weight losses equivalent
to two or one coordinated water molecules. Binary complexes of Cr(III)
with both ligands and Fe(III)-L2, have one coordinated H2O. At higher
temperatures (> 300◦C), all binary and ternary complexes display rapid
and successive weight loss steps revealing their decomposition yielding
the corresponding metal oxides or pyrophosphates.
The reflectance electronic spectra of the absorption bands recorded
in ethanolic solutions for solid binary complexes display an absorption
band at 320 nm (for M-L1) and at 276 nm (for M-L2). These bands are
assigned to the intramolecular charge transfer (CT) transition from
the ligand to the unfilled orbital localized on the metal ion (L →M).
The bands are broad and ascribed to combination of some transitions,
4
4
4
4
4
4
namely, A2 → T2, A2 → T1(F), A2 → T1(P) for Cr3+ chelates, and
4T1 → E1, A1,4T1 → T2 for Fe3+ and Mn2+ chelates. These transi-
4
4
4
tions indicate octahedral stereochemistry for all complexes except those

2436
A. A. M. Gad et al.
binary complexes of manganese which attain the tetrahedral structure.
This result is evidenced from the colors and the absorptivity values,
which are deeper and greater, respectively for binary (tetrahedral) and
the reverse for ternary (octahedral). It is worth mentioning that binary
Fe-L2 and ternary Fe-L1-L2 complexes show a small splitting in their
electronic spectra appearing at 340 nm and indicating the occurrence
of some distortion in t2g orbitals due to Jahn Teller effect.32
Generally, the data of microanalyses, conductivity measurements,
1H NMR, IR, and reflectance spectra and TGA analysis, reveal that the
neutral ligand is coordinated to the metal ion as bidentate P N and
P O or C S forming neutral monomeric metal chelates as represented
by Scheme 2.
SCHEME 2

Physicochemical and Thiourea Monophosphazene Derivatives
2437
The structures of the complexes are suggested depending on the fol-
lowing facts:
1) in agreement with a previous work,12,13 these ligands can be coordi-
nated with transition metal ions via other atoms than phosphorus
atom;
2) the nitrogen atom of the P N are stronger bases compared to that
of P-NH, due to the hyper conjugation between nitrogen lone pair
and adjacent P N σ∗ orbitals; and
3) the proposed structure has less steric hindrance, thus the less ster-
ically hindered substituents allow easier complexation and leading
to its higher stability constant.
Conclusion
Stability and structural characterization of some metal complexes of
bidentate phosphorus ligands possessing varying steric and environ-
ments have been determined. From the previous discussion, it is con-
cluded that the ligands bond to metal ions through the imino nitrogen
atom of P N and oxygen atom of P O (CDPZ) or sulfur atom of C S
(TMPZ).
The stereochemistry of the ligand appears to play a significant role
in determination of its coordination sites. In addition, steric hinderance
and greater π-accepting nature of the phosphorus centers governs the
stability of the formed chelates. The increasing stability constants of
the ternary complexes can be attributed to the intramolecular stack-
ing interaction.33 Thus, the cyclodiphosphazane (L2) is rigid and is also
a good π-acceptor molecule, while the ring system of thiomonophosp-
hazene (L1) is attached to side chain. When L1 and L2 coordinate to the
same metal ion, the L1 reaches easily to the rigid L2 molecule producing
intramolecular aromatic ring stacking.34
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