The bond angle in a molecule of ammonia (NH3) is 107 degrees so why, when part of a transition metal complex is the bond angle 109.5 degrees.

Ammonia is based off a tetrahedral shape, the central Nitrogen atom has 4 valence (outer) pairs of electrons, 3 in covalent bonds with Hydrogen atoms and one "lone pair" which are not bonded. The tetrahedral shape has bond angles of 109.5 degrees, but the lone pair exists closer to the nucleus than the bonding pairs and has a greater repulsive effect than the three bonding pairs, therefore pushing them closer together and decreasing the bond angle by 2.5 degrees. When in a transition metal complex the lone pair is co-ordinately (dative covalently) bonded to the central metal atom to form the transition metal complex. This means that all four valence pairs are bonding and have therefore equal repulsive effects, meaning that the bond angles are equal at 109.5 degrees.