why? .There occurs much more frequent metal-metal bonding in compounds of heavy Transition

metals.

due to the nature of unpaired electrons

Due to variable oxidation states

because these have smaller size when compared to what they should have for their atomic and mass number due to lanthanoid contraction thus they can produce better metallic bonds

It is due to presence of unpaired electrons in the (n-1)d shell they have tendency to make strong metallic bond by covalent nature of bonds

The frequent metal-metal bonding in compounds of heavy transition elements is due to their high enthalpy of atomization.
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.