There occurs much more frequent metal- metal bonding in compounds of heavy transition elements.give reason.
The transition metals (with the exception of Zn, Cd and Hg) are very much hard and have low volatility. Their melting and boiling points are high. The high melting points of these metals are attributed to the involvement of greater number of electrons from (n-1)d in addition to the ns electrons in the interatomic metallic bonding. In any row the melting points of these metals rise to a maximum at d5 except for anomalous values of Mn and Tc and fall regularly as the atomic number increases. They have high enthalpies of atomisation. The maxima at about the middle of each series(as shown in figure below) indicate that one unpaired electron per d orbital is particularly favourable for strong interatomic interaction. In general, greater the number of valence electrons, stronger is the resultant bonding. Since the enthalpy of atomisation is an important factor in determining the standard electrode potential of a metal, metals with very high enthalpy of atomisation (i.e., very high boiling point) tend to be noble in their reactions.
Hence, the metals of the second and third series have greater enthalpies of atomisation than the corresponding elements of the first series. Thus, this is an important factor in accounting for the occurrence of much more frequent metal metal bonding in compounds of the heavy transition metals.