The stability of a complex relies on

1. Nature of central ion: the word nature means the charge density on the central ion i.e. greater is the charge density or larger the charge/radius ratio more is the stability of a complex. For example, out of complexes of Fe²⁺ and Fe³⁺, are more stable.

For example,
                                 
Fe³⁺ + 6CN^-    [Fe(CN)₆]^3-; K = 1.2 × 10³¹
                                  
Fe²⁺ + 6CN^-    [Fe(CN)₆]^4- ; K = 1.8 × 10⁶

For the ions which carry the same charge the one with a smaller size the more stable complexes. For example, among Cu²⁺, Ni²⁺, Co₂⁺, Fe²⁺ complexes as the size of copper is the smallest therefore it gives the most stable complexes.

2. Nature of ligand

(a) Basic character of ligands: more basic is a ligand, greater is the ease with which it can donate its electrons and therefore more is the stability of the complex. For example, the complexes involving F- ions are more stable than those involving Cl^- ions or Br^- ions.

(b) Charge on ligands: for charged ligands, the higher the charge and the smaller their size, the more stable are the complexes.

(c) Chelate effect: also complexes containing chelate rings are usually more stable than similar complexes containing no rings. This is termed as chelate effect.

For example,
                       
Ni²⁺(aq) + 6NH₃(aq) ? [Ni(NH₃)₆]²⁺(aq) log = 7.99
                       
Ni²⁺(aq) + 3 en(aq) ? [Ni(en)₃]²⁺(aq) log = 18.1

3. Crystal field effects (Irving-William Order): the stability of high spin complexes of the ions between Mn²⁺ and Zn²⁺ with a given ligand frequently vary in the order:

Mn²⁺ < Fe⁺² < Co⁺² < Ni²⁺ < Cu⁺² > Zn⁺²

This order is also called natural order and is consistent with charge to radius ratio concept. Radii of these ions are:

Mn⁺² (0.91 Å); Fe⁺² (0.83 Å); Co⁺² (0.82 Å); Ni⁺² (0.78 Å); Cu⁺² (0.69 Å); Zn⁺² (0.74 Å)

4. Class a and class b metals: chatt and Ahrland have classify the metals into three groups i.e. a, b and borderline on the basis of their electron acceptor properties.

(i) Class a metals: H, group 1 (alkali metals), group 2 (alkaline earths), the elements Sc to Cr; Al to Cl; Zn to Br, In, Sb, Sn, I. these metals form stable complexes with ligands containing N, O and F groups.

(ii) Class b metals: Rh, Pd, Ag, Ir, Pt, Au, Hg.

These elements have fully filled d-orbitals and therefore form more stable complexes with ligands whose donor atoms are heavier member of N, O and F groups.

(iii) Border line metals: elements from Mn to Cu, Tl to Po, Mo, Te, Ru, W, Re, Os, Cd. These do not follow a particular stability order.

The stability order of complexes of these elements with ligands follows the following order:

Class a elements:

F^- > Cl^- > Br^- > I^- > O > S > Se > Te > N >> P > As > Sb > Bi

Class b elements:    

F^- < Cl^- Br^- < I^- < O << S ≈ Se ≈ Te < N << P < As < Sb < Bi