Give reasons for (a) \(\mathrm{Pd}(\mathrm{CO})_{4}\) and \(\mathrm{Pt}(\mathrm{CO})_{4}\) are not stable but \(\mathrm{Ni}(\mathrm{CO})_{4}\) is stable. (b) \(\mathrm{Ni}(\mathrm{CO})_{2} \mathrm{Cl}_{2}\) is not formed but \(\mathrm{Pd}(\mathrm{CO})_{2} \mathrm{Cl}_{2}\) is stable.

Transition metal complexes often strive for a particular stability known as the 18-electron rule. This guideline suggests that a complex is most stable when the total count of valence electrons from the metal and surrounding ligands equals 18, the electron configuration of a noble gas.
This rule can be better understood by considering that the valence shell of transition metals consists of the ns, (n-1)d, and np orbitals, with a capacity of holding up to 18 electrons. Reaching this 'magic number' provides stability akin to the electron configuration of inert gases.
In the context of carbonyl complexes, like \(\text{Ni}(\text{CO})_{4}\), nickel provides 10 electrons, and each carbon monoxide ligand contributes two electrons, together reaching the stable 18-electron configuration. However, heavier metals such as palladium and platinum also bring in 10 d-electrons, yet they exhibit relativistic effects that modulate electron density and orbitals, eschewing the straightforward application of the 18-electron rule and potentially undermining the stability of \(\text{Pd}(\text{CO})_{4}\) and \(\text{Pt}(\text{CO})_{4}\).

Transition metal complexes are formed when transition metal ions bond with ligands—atoms, ions, or molecules that can donate a pair of electrons. In such complexes, the metal atom's d orbitals provide a region of relatively low energy where ligands can donate their electron pairs – an interaction that is the foundation of the complex's formation.
Furthermore, the geometry of the complex plays a critical role in its overall stability. For instance, the square planar configuration of \(\text{Pd}(\text{CO})_{2} \text{Cl}_{2}\) is inherently stable due to its electronic arrangement and spatial considerations despite not meeting the 18-electron rule. This showcases that, while the 18-electron rule is a helpful guideline, it is the interplay of various factors such as electronic configuration, geometry, and the nature of the metal-ligand bond, that truly determines the stability of transition metal complexes.

Ligands contribute to the stability of transition metal complexes by donating electron pairs to the metal, which helps fulfill the metal's valence electron shell. Different types of ligands contribute varying numbers of electrons.
For example, in the step by step solutions mentioned, CO is a strong field ligand that donates two electrons, whereas Cl is a weaker field ligand donating only one electron. The ability of ligands to donate electrons can also influence the geometry of the complex. In some cases, as with \(\text{Pd}(\text{CO})_{2} \text{Cl}_{2}\), the nature of the ligands and the resulting geometry can allow a complex to be stable even if it doesn't conform to the 18-electron rule, illustrating the significant impact that the 'choice' of ligand exerts on the overall properties of a transition metal complex.