The force field for the cellular ligand field stabilisation
energy/molecular mechanics (CLFSE/MM) method has been applied to 28
transition-metal complexes. Computed and experimental structures are
compared for 12
ML
x
Cl
6-
x
species
(M = Co
III
or Ni
II
;
L = amine donor; x = 6 or 4), 12
MA
x
B
6-
x
compounds
(M = Ni
II
or Cu
II
;
A = imine, B = amine;
x = 6, 4 or 3), one five-co-ordinate
copper(
II) imine–amine complex and three
four-co-ordinate copper(
II) imine and imine–amine
molecules. For π-bonding ligands a stronger donor interaction is
associated with a larger positive value of the CLF
e
π
parameter but, due to the use of a crystal
field type barycentre, the CLFSE actually goes up. The CLFSE thus has
the wrong form for treating the π contributions to bond stretching
and distance-dependent e
π
parameters are
inappropriate. However, the desired bond lengths can be obtained by
modifying the Morse function and e
σ
terms. The
π contribution to the L–M–L angle bending operates in
the correct sense but is small and can also be accommodated by altering
the magnitude of e
σ
. For asymmetric π
interactions
(e
πx
≠
e
πy
) there is no effect on the
M–L torsional potential for low-spin d
6,
high-spin d
8 and d
9
configurations where the π-symmetry d orbitals are completely filled.
Hence, only the σ-bonding contributions to the CLFSE are retained.
This approach still gives good agreement with experimental structures,
even for formally π-bonding ligands, with average root-mean-square
errors in M–L lengths and L–M–L angles of about
0.02 Å and 3° for Co
III
, Ni
II
and four co-ordinate Cu
II
, excluding
[Cu(bipy)
2
]
2+
(bipy = 2,2′-bipyridyl), and about 0.05
Å and 4° respectively for six-co-ordinate
Cu
II
, excluding [Cu(terpy)
2
]
2+
(terpy = 2,2′∶6′,2″
-terpyridyl). The subtle interplay between the axial Ni–Cl and
equatorial Ni–N distances in
trans-[NiN
4
Cl
2
] macrocyclic species is
reproduced for the first time by an MM-based approach. However, the
model appears to give relatively poor agreement for
[Cu(bipy)
2
(NH
3
)]
2+
,
[Cu(terpy)
2
]
2+
and
[Cu(bipy)
2
]
2+
. For the five-co-ordinate complex
this is due to the intrinsic plasticity of five-co-ordinate
copper(
II) species. The energy difference between the
limiting trigonal-bipyramidal and square-pyramidal geometries is only a
few kcal mol
-1
. For
[Cu(terpy)
2
]
2+
the limiting geometries of
tetragonally elongated and compressed octahedra are also within a few
kcal mol
-1
although the present set of parameters
overestimates the ligand contribution and predicts a compressed
geometry. The calculated structure of
[Cu(bipy)
2
]
2+
is too flat but for four-co-ordinate
species it is shown, using [CuCl
4
]
2-
as an
example, that there are several ways to induce a tetrahedral distortion.
The most satisfactory method is to include charges on Cu and the ligand
donors whereupon the geometries of [CuCl
4
]
2-
and [Cu(bipy)
2
]
2+
distort to the required
flattened tetrahedral structures.