The bond formed between 2 atoms by sharing a pair of electrons, provided entirely by one of the combining atoms but shared by both is called a coordinate bond or dative bond, e,.g., ammonium ion, hydronium ion
conditions for the formation of coordinate bond
1) one of the 2 atoms must have at least one lone pair of electrons e. g, ammonia, water
2) another atom should be short of at least a lone pair of electrons, e. g., hydrogen ion