Chapter 3, Problem 3.17P

Explain the following:
a. ${\text{PCI}}_{\text{5}}$ is a stable molecule, but ${\text{NCl}}_{\text{5}}$ is not.
b. ${\text{SF}}_{\text{4}}$ and ${\text{SF}}_{\text{6}}$ are known, but ${\text{OF}}_{\text{4}}$ and ${\text{OF}}_{\text{6}}$ are not.