Which of the following is non-conducting?

Which one of the following metal complexes is most stable?

In the isoelectronic series of metal carbonyls,the $CO$ bond strength is expected to increase in the order:

The stepwise stability constants of a complex are given below. What is its overall reaction stability constant $\beta_4$ ?
$M + L \rightleftharpoons ML ; K_1 = 1.0 \times 10^4$
$ML + L \rightleftharpoons ML_2 ; K_2 = 1.0 \times 10^3$
$ML_2 + L \rightleftharpoons ML_3 ; K_3 = 1.0 \times 10^3$
$ML_3 + L \rightleftharpoons ML_4 ; K_4 = 1.0 \times 10^2$
(Overall reaction : $M + 4L \rightleftharpoons ML_4$)

What is the color of a cobalt chloride solution?

Which of the following complexes is octahedral,diamagnetic,and the most stable?