In general, ligands are viewed as donating electrons andelectrostatic molecules to the central atom. Bonding is oftendescribed using the formalisms of molecular orbital theory. Ingeneral, electron pairs occupy the HOMO (Highest Occupied MolecularOrbital) of the ligands. Ligands and metal ions can be ordered in many ways; one rankingsystem focuses on ligand \'hardness\' (see also hard/soft acid/basetheory). Metal ions preferentially bind certain ligands. Ingeneral, \'hard\' metal ions prefer weak field ligands, whereas\'soft\' metal ions prefer strong field ligands. According to themolecular orbital theory, the HOMO of the ligand should have anenergy that overlaps with the LUMO (Lowest Unoccupied MolecularOrbital) of the metal preferential. Metal ions bound tostrong-field ligands follow the Aufbau principle, whereas complexesbound to weak-field ligands follow Hund\'s rule. Binding of the metal with the ligands results in a set ofmolecular orbitals, where the metal can be identified with a newHOMO and LUMO (the orbitals defining the properties and reactivityof the resulting complex) and a certain ordering of the 5d-orbitals (which may be filled, or partially filled withelectrons). In an octahedral environment, the 5 otherwisedegenerate d-orbitals split in sets of 2 and 3 orbitals (for a morein depth explanation, see crystal field theory). The energy difference between these 2 sets of d-orbitals iscalled the splitting parameter, o. The magnitudeof o is determined by the field-strength of theligand: strong field ligands, by definition, increaseo more than weak field ligands. Ligands can now besorted according to the magnitude of o (see thetable below). This ordering of ligands is almost invariable for allmetal ions and is called spectrochemical series. For complexes with a tetrahedral surrounding, the d-orbitalsagain split into two sets, but this time in reverse order: The energy difference between these 2 sets of d-orbitals is nowcalled t. The magnitude of t issmaller than for o, because in a tetrahedralcomplex only 4 ligands influence the d-orbitals, whereas in anoctahedral complex the d-orbitals are influenced by 6 ligands. Whenthe coordination number is neither octahedral nor tetrahedral, thesplitting becomes correspondingly more complex. For the purposes ofranking ligands, however, the properties of the octahedralcomplexes and the resulting o has been of primaryinterest. Solution In general, ligands are viewed as donating electrons andelectrostatic molecules to the central atom. Bonding is oftendescribed using the formalisms of molecular orbital theory. Ingeneral, electron pairs occupy the HOMO (Highest Occupied MolecularOrbital) of the ligands. Ligands and metal ions can be ordered in many ways; one rankingsystem focuses on ligand \'hardness\' (see also hard/soft acid/basetheory). Metal ions preferentially bind certain ligands. Ingeneral, \'hard\' metal ions prefer weak field ligands, whereas\'soft\' metal ions prefer strong field ligands. .

1. In general, ligands are viewed as donating electrons andelectrostatic molecules to the central
atom. Bonding is oftendescribed using the formalisms of molecular orbital theory. Ingeneral,
electron pairs occupy the HOMO (Highest Occupied MolecularOrbital) of the ligands.
Ligands and metal ions can be ordered in many ways; one rankingsystem focuses on ligand
'hardness' (see also hard/soft acid/basetheory). Metal ions preferentially bind certain ligands.
Ingeneral, 'hard' metal ions prefer weak field ligands, whereas'soft' metal ions prefer strong
field ligands. According to themolecular orbital theory, the HOMO of the ligand should have
anenergy that overlaps with the LUMO (Lowest Unoccupied MolecularOrbital) of the metal
preferential. Metal ions bound tostrong-field ligands follow the Aufbau principle, whereas
complexesbound to weak-field ligands follow Hund's rule.
Binding of the metal with the ligands results in a set ofmolecular orbitals, where the metal can be
identified with a newHOMO and LUMO (the orbitals defining the properties and reactivityof the
resulting complex) and a certain ordering of the 5d-orbitals (which may be filled, or partially
filled withelectrons). In an octahedral environment, the 5 otherwisedegenerate d-orbitals split in
sets of 2 and 3 orbitals (for a morein depth explanation, see crystal field theory).
The energy difference between these 2 sets of d-orbitals iscalled the splitting parameter, o. The
magnitudeof o is determined by the field-strength of theligand: strong field ligands, by
definition, increaseo more than weak field ligands. Ligands can now besorted according to the
magnitude of o (see thetable below). This ordering of ligands is almost invariable for allmetal
ions and is called spectrochemical series.
For complexes with a tetrahedral surrounding, the d-orbitalsagain split into two sets, but this time
in reverse order:
The energy difference between these 2 sets of d-orbitals is nowcalled t. The magnitude of t
issmaller than for o, because in a tetrahedralcomplex only 4 ligands influence the d-orbitals,
whereas in anoctahedral complex the d-orbitals are influenced by 6 ligands. Whenthe
coordination number is neither octahedral nor tetrahedral, thesplitting becomes correspondingly
more complex. For the purposes ofranking ligands, however, the properties of the
octahedralcomplexes and the resulting o has been of primaryinterest.
Solution
In general, ligands are viewed as donating electrons andelectrostatic molecules to the central
atom. Bonding is oftendescribed using the formalisms of molecular orbital theory. Ingeneral,
electron pairs occupy the HOMO (Highest Occupied MolecularOrbital) of the ligands.
Ligands and metal ions can be ordered in many ways; one rankingsystem focuses on ligand

2. 'hardness' (see also hard/soft acid/basetheory). Metal ions preferentially bind certain ligands.
Ingeneral, 'hard' metal ions prefer weak field ligands, whereas'soft' metal ions prefer strong
field ligands. According to themolecular orbital theory, the HOMO of the ligand should have
anenergy that overlaps with the LUMO (Lowest Unoccupied MolecularOrbital) of the metal
preferential. Metal ions bound tostrong-field ligands follow the Aufbau principle, whereas
complexesbound to weak-field ligands follow Hund's rule.
Binding of the metal with the ligands results in a set ofmolecular orbitals, where the metal can be
identified with a newHOMO and LUMO (the orbitals defining the properties and reactivityof the
resulting complex) and a certain ordering of the 5d-orbitals (which may be filled, or partially
filled withelectrons). In an octahedral environment, the 5 otherwisedegenerate d-orbitals split in
sets of 2 and 3 orbitals (for a morein depth explanation, see crystal field theory).
The energy difference between these 2 sets of d-orbitals iscalled the splitting parameter, o. The
magnitudeof o is determined by the field-strength of theligand: strong field ligands, by
definition, increaseo more than weak field ligands. Ligands can now besorted according to the
magnitude of o (see thetable below). This ordering of ligands is almost invariable for allmetal
ions and is called spectrochemical series.
For complexes with a tetrahedral surrounding, the d-orbitalsagain split into two sets, but this time
in reverse order:
The energy difference between these 2 sets of d-orbitals is nowcalled t. The magnitude of t
issmaller than for o, because in a tetrahedralcomplex only 4 ligands influence the d-orbitals,
whereas in anoctahedral complex the d-orbitals are influenced by 6 ligands. Whenthe
coordination number is neither octahedral nor tetrahedral, thesplitting becomes correspondingly
more complex. For the purposes ofranking ligands, however, the properties of the
octahedralcomplexes and the resulting o has been of primaryinterest.