Chemistry Reference
In-Depth Information
[NiCl
4
]
2-
[Ni(CN)
4
]
2-
Cl
NC
CN
Ni
Ni
Cl
NC
CN
Cl
Cl
square planar
tetrahedral
no unpaired electrons
diamagnetic
two unpaired electrons
paramagnetic
Figure 4.10
Examples of d
8
nickel(II) complexes adopting square planar or tetrahedral geometry, depending on
the type of ligand. The square planar complex has no unpaired electrons (diamagnetic) whereas the
tetrahedral complex has two unpaired electrons (paramagnetic), allowing easy identification (energy
splitting diagrams not to scale).
the d
8
Ni(II) ion can form octahedral, tetrahedral and square-planar complexes, with strong-
field ligands tending to favour square-planarity. They are distinguished readily from their
colours and spectroscopic properties; for example, octahedral Ni
N
6
complexes are purple
high-spin species and square-planar Ni
N
4
complexes yellow low-spin species (here,
N
represents an N-donor ligand, not an isolated atom). This ambivalence is a reminder that
the square planar geometry can be reached by removal of two
trans
-disposed ligands from
around an octahedron, as well as by bond angle distortion of a tetrahedron. The structural
ambivalence of d
8
Ni(II) is also exemplified in Figure 4.10, where different ligands lead
to different geometry; apart from spectroscopic differences, magnetic properties of square
planar and tetrahedral complexes differ, the latter alone having unpaired electrons.
Since square-planar complexes are essentially two-dimensional compounds, it is not a
surprise to find other molecules coming into reasonably close but still nonbonding po-
sitions (distances
300 pm) above and below the plane in the solid state and in solution,
including metal-solvent and even some cases of metal-metal interaction. The square planar
cation-anion pair [Pt(NH
3
)
4
]
2
+
[PtCl
4
]
2
−
(called, in earlier times, Magnus' green salt) for
example, stack cationic and anionic complexes in alternating positions in their crystalline
lattice, with a M
325 pm (Figure 4.11).
The separation is close enough to have an effect on physical properties, with the green
colour indicating some weak metal-metal interaction, since another form of the salt with a
very long separation of over 500 pm and no semblance of interaction is pink, colour arising
...
M separation of
∼
Cl
NH
3
Cl
NH
3
Cl
Cl
H
3
N
Cl
H
3
N
Cl
Pt
Pt
Pt
Pt
Pt
Cl
H
3
N
Cl
H
3
N
Cl
Cl
NH
3
Cl
NH
3
Cl
Figure 4.11
Stacking of a d
8
platinum(II) complex in the solid state leads to weak Pt
···
Pt interaction, influencing
physical properties.
