why metal metal bonding is more frequent for the 4d and the 5d series of transtion metals than that for the 3d series????????????

3 d elements have smaller size as compare to the 4d and 5d elements.

Due to the small size, valence electrons of 3d metals are strongly bonded with the nucleus and thus they are not free for metal metal bonding.

While in 4d and 5d elements the valence electrons are not so strongly bonded with nucleus and hence have greater tendency for metal metal bonding.