Transition metals and their ions show strong tendency for complex formation. The cations of transition elements (d-block elements) form complex ions with certain molecules containing one or more lone-pairs of electrons, viz., CO, NO, NH₃ etc., or with anions such as, F^–, Cl^–, CN^– etc. A few typical complex ions are,

[Fe(CN)]^4–, [Cu(NH₃)₄]²⁺, [Y(H₂O)₆]²⁺,

[Ni(CO)₄], [Co(NH₃)₆]³⁺ [FeF₆]^3–

Explanation : This complex formation tendency is due to,

(i) Small size and high nuclear charge of the transition metal cations.

(ii) The availability to vacant inner d-orbitals of suitable energy.

Formation of interstitial compounds : Transition elements form a few interstitial compounds with elements having small atomic radii, such as hydrogen, boron, carbon and nitrogen. The small atoms of these elements get entrapped in between the void spaces (called interstices) of the metal lattice. Some characteristics of the interstitial compound are,

(i) These are non-stoichiometric compounds and cannot be given definite formulae.

(ii) These compounds show essentially the same chemical properties as the parent metals, but differ in physical properties such as density and hardness. Steel and cast iron are hard due to the formation of interstitial compound with carbon. Some non-stoichimetric compounds are, VSe_0.98 (Vanadium selenide), Fe_0.94O and titanium nitride.

Explanation : Interstital compounds are hared and dense. This is because, the smaller atoms of lighter elements occupy the interstices in the lattice, leading to a more closely packed structure. Due to greater electronic interactions, the strength of the metallic bonds also increases.