NOTIZEN
E.P.R . determ ination of the hybridisation and
the H-N-H bond angle of ammonia in
[C u(N H 3)4][PtC l4]
J. B.
The unpaired electron has be shown to be in the
b\a molecular orbital whose wavefunction is
a1
X
F * b i g + a ( d x 1- y !) — —2
( ° i + °2 + °3 + ° i ) •
(1 )
R aynor
The corresponding bonding molecular orbital is given
by
D epartm ent of Chemistry, University of Leicester,
L eicester L EI 7RH, England
(Z. N aturforschg. 24 b, 775 [1969] ; eingegangen am 9. April 1969)
F r i t z and K e l l e r 1 have made a very detailed in­
vestigation of the e.p.r. parameters of [Cu(NH3) 4]
[PtCl4] substituted in the host lattice of M a g n u s’s
salt [Pt(N H 3) 4] [PtCl4] . From the g-tensor and the
metal hyperfine coupling tensor, the bonding para­
meters have been obtained, fn this note, f wish to
point out an error in the analysis of the nitrogen superhyperfine tensor deduced from the powder, and with
the modified values, show that the interaction of the
unpaired electron with the nitrogen atom of the am­
monia ligands may give information about the hybridi­
sation of the nitrogen lone pair electrons and hence
the ff —N —ff bond angle.
The [Cu(NH3) 4] 20 ion has strictly square planar
symmetry in this lattice (ignoring the hydrogen atoms).
fn D_th symmetry, when the applied field lies parallel
to the z-axis, the nitrogen superhyperfine splittings
observed are ß ( 14N) since the principal directions of
the 14N tensor lie parallel to the Cu —N bonds. Like­
wise, the features attributable to 14N when the applied
field is perpendicular to the z-axis are not B (14N ).
When the applied field is parallel to the x-axis, the two
nitrogen atoms along the x-axis contribute to A (14N)
but the other two, along the ?/-axis, contribute to
Z?(14N ). A similar argument applies when the field is
along y. Hence the turning points of the powder spec­
trum occur at h ( A + B ) . The superhyperfine splittings
measured by K r i t z and K e l l e r from their powder
spectrum were A = 1 0 .5 x l0 -4 cm-1 and ß = 1 3 x l0 -4
cm- 1 . These measured values should be reassigned as
follows: 5 = 1 0 .5 x l0 -4 cm-1 (= 1 0 .2 gauss) and
i (A + B ) = 13 x 10~4 cm-1 (= 1 3 .6 gauss), from which
.4 = 17.0, B = 10.2 and AiS0 = f2.5 gauss. The dipolar
contribution to the tensor must be subtracted (using
the point-dipole approximation, A d =
2 gN /?N
r3 ~ = 0.47
gauss, where r is the Cu —N distance2). The principal
value of the 14N superhyperfine tensor is thus 4.0 gauss.
The 2 p-character of the unpaired electron cp2 =
11.7% and the 2 s-character cs2 =
12.5
q
4.0
34.1
= 2 .3 per
cent 3.
1 H. P. F r i t z and H. J. K e l l e r , Z. Naturforschg. 20 b, 1145
[1965].
2 M. A t o j i , J. W. R i c h a r d s o n , and R . E. R u n d l e , J. Amer.
chem. Soc. 79. 3017 [1957].
3 M. C. R . S y m o n s , Advances in Physical Organic Chemistry,
ed. G o l d , Academic Press, London-New York 1962, Vol. 1,
p. 332.
X
F b \g =
( d x 1- ^ )
+
2
( ° i + °2 + °3 + ° i ) •
(2 )
Since all four nitrogen atoms are equivalent, the wavefunctions of the lone-pair electrons on the nitrogen
atoms of the NH3 group are equal, thus
Oj = O2 = 0‘j = 04
o.
(3)
The wavefunction of o is itself a linear combination
o = cs
+ Cp W2p .
(4)
where W-zs and W2p are the nitrogen 2 s and 2 p orbi­
tals from which the hybridisation 5 pn may be de­
duced, viz.
n = /3-2 = —CPo'
(5)
4 has shown that if one assumes that the
bonds formed from hybridisation of s- and p-orbitals
on an atom are orthogonal to each other, then it is pos­
sible to estimate the bond angle from the hybridisation
ratio. For ligands like NH3 with C3 V symmetry, the
bond angle H —N —H ( 0 ) is given by
C o u ls o n
0
=
1.5
2 A2+ 3
(6)
In the case of [Cu(NH3) 4] [PtCl4], A2 = 5.1 and it fol­
lows that H —N —H = 112° 45
This value is greater than that for NH3 (107°). An
increase of this magnitude is to be expected since the
lone pair electrons will be further away from the nitro­
gen atom because Cu20 is more electronegative. This
decreases the Cu —N bond-pair — N —H bond-pair
repulsion and allows the H —N —H bond angle to
relax 5.
ft should be emphasised that this method of cal­
culating bond angles from the spin distribution of un­
paired electrons is only applicable if the wave function
of the unpaired electron is related to the wavefunction
of the bonding electrons. This is so in this case be­
cause corresponding bonding and antibonding orbitals
are involved. The importance of this simple method of
determining bond angles is highlighted by the experi­
mental difficulties of measuring such bond angles in
this type of complex by neutron diffraction techniques.
A . C o u l s o n , Volume Commemoratif V i c t o r H e n r i ,
C ontributions ä l’Etude de la Structure M oleculaire, 1948,
p. 15; see also P. W. A t k i n s and M. C . R. S y m o n s , “The
structure of Inorganic R adicals” , Elsevier, Am sterdam 1967,
p. 257.
C . E. M e l l i s h and J. W. L i n n e t t , Trans. Faraday Soc. 50,
657 [1954],
4 C.
5
Unauthenticated
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