B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 74 (2009) 1016–1024
1021
Table 6
g and A value of the solution* ESR spectra of VO(IV) complexes.
Complexes
g
||
g
⊥
|g|
A
||
(× 10−4 cm−1
)
A
(× 10−4 cm−1
⊥
)
|A| (× 10−4 cm−1
)
[VO(L1)2H2O]
[VO(L2)2H2O]
[VO(L3)2H2O]
1.93
1.93
1.92
1.97
1.97
1.99
1.96
1.96
1.97
157.94
157.89
158.74
75.23
75.33
77.83
102.80
102.85
104.80
||
⊥
|A| = 1/3(A + 2A ).
||
⊥
orbital of a molecule which exists in distorted octahedral geometry
[26–28]. The molecular orbital coefficient ˛2 or (ˇ2*)2 (covalent in-
plane -bonding) and ˇ2 or (ˇ1*)2 (covalent in-plane -bonding)
were calculated by use the solution ESR spectral data and values
are given in Table 7.
G value calculated by using the following equations:
|| − 2.002
⊥ − 2.002
g
G =
g
If G < 4.0, the ligand forming the complex is regarded as a
strong field ligand. The G value of the complexes [VO(HL1)2H2O],
[VO(HL2)2H2O] and [VO(HL3)2H2O] at room temperature are 3.83,
3.83 and 3.47 and at LNT are 2.42, 2.42 and 3.27, respectively, indi-
cating that the Schiff base ligands are strong field ligands and the
metal–ligand bonding in these complexes is covalent.
The bonding parameters were calculated by using the following
equations [30,31]:
The value of the in-plane -bonding coefficient (ˇ2*)2 gener-
ally follows the donor strength of the ligand, i.e. (ˇ2*)2 decrease
as covalent bonding increase. This parameter shows larger varia-
tion in present VO(IV) Schiff base complexes. The dependences of
K on (ˇ2*)2 arise from participation of the empty 4s orbitals on
the metal in -bonding to the ligands. The empty 4s orbital of the
metal can overlap with the filled -levels of the basal ligands as
effectively as can the dx
orbital of the metal. The molecular
2
2
−y
(A|| − A ) = 6/7 ( ∗)2P − (ge − g )P + 5/14(ge − g )P
orbital formed from the 4s orbital should put partial 4s density in a
filled bonding orbital which in turn should undergo spin polariza-
tion by the dxy electron. The extent of the ligand to metal interaction
⊥
||
⊥
2
8(ˇ1∗)2(ˇ2∗)2ꢂ
ꢃE1dxy → dx
should be related to both energy of antibonding dx
energy level
2
2
−y
ge − g
=
||
and its orbital coefficient. The 4s contribution to K should propor-
tional to the metal electron density in the filled orbital that contains
contribution of 4s orbital. The delocalization in the -system of
the complex is expressed by the bonding coefficient for the dx
2
2
−y
2(ˇ2∗)2(eꢄ∗)2ꢂ
2
2
−y
ge − g
=
⊥
ꢃE2dxy → dxz,yz
level.
The origin of the isotropic constant term K which is related to
the amount of unpaired electron density on the vanadium nucleus
dium nucleus and does not mix with the metal 4s orbital (in C4V
symmetry), there is no direct way of putting unpaired electron
density on the nucleus. The nonzero value of K must then arise
from an indirect mechanism. Mc Gravey [29] suggests that varia-
tion can be explained by involving a spin polarization mechanism.
The unpaired electron in the dxy orbital formally creates unpaired
electron density in filled 2s and 3s orbitals of the vanadium. In
the absence of covalent bonding and 4s mixing, spin polarization
should remain constant for all vanadium complexes and be equal
to the free ion value, KO. Taking into account covalent bonding, K
shows depend on the d-orbital population for the unpaired elec-
tron, K∼ (ˇ2*)2KO.
|A| = − PK − (ge − |g|)P
ge = 2.0023, A , A and |A| are taken to be negative [30], P is the
||
⊥
free ion dipole term = 128 × 10−4 cm−1, ꢂ = spin-orbit coupling con-
stant = 170 cm−1, ꢃE1 = electronic transition energy of dxy → dx
2
2
−y
ꢃE2 = electronic transition energy of dxy → dxz,yz
.
3.8. Thermal study
In the present studies of the metal complexes of VO(IV),
we select non-isothermal mode for TGA, DTA and DSC tech-
niques. From the TGA curves of complexes [VO(L1)2H2O] and
[VO(L3)2H2O], they have been observed that there are four steps
and [VO(L2)2H2O], [VO(L4)2H2O], [VO(L5)2H2O] and [VO(L6)2H2O],
they have been observed two steps, in the decomposition of VO(IV)
cal exhibit two endothermic peaks in the range of 120–180 ◦C and
305–430 ◦C corresponding to the detachment of coordinated H2O
which are shown in Figs. 6 and 7. The first mass loss 3.25–4.00%
(2.81–3.89% cala.) up to 180 ◦C is in good agreement with loss of
oxide V2O5 was formed. The activation energy ꢃE*calculated by
Broido graphical method and Broido plots are shown in Fig. 8. From
the DSC curve, we found out the ꢃH* and ꢃS* and their values are
given in Table 8.
An analysis of the bonding coefficients in the three com-
plexes reveals that the strength of the equatorial plane -bonding
(ˇ1*)2 increases in the order 1.29 for [VO(L1)2H2O] < 1.32 for
[VO(L2)2H2O] < 1.63 for [VO(L3)2H2O] and the equatorial in-plane
bond (ˇ2*)2 strength stays about the same [31]. The (e*)2, the
bonding coefficient for dXZ and dYZ orbitals, measures the covalency
of the oxovanadium bonds. It also indirectly shows the strength of
in-plane ligands, since the stronger the in-plane donor atom, the
less covalent is the oxovanadium bond.
Table 7
Bonding parameter of VO(IV) complexes.
Complexes
(ˇ2*)2
(ˇ1*)2
(e*)2
K
3.8.1. Broido’s method
[VO(L1)2H2O]
[VO(L2)2H2O]
[VO(L3)2H2O]
0.683
0.681
0.645
<1
<1
<1
0.974
0.967
0.976
0.760
0.760
0.786
A Broido has suggested a simple and sensitive graphical method
for the treatment of TGA data. According to this method, the weight
at any time t (wt.) is related to the fraction of initial molecules not