If the directors of a company make a decision, which later on proves not to be a good decision and causes the company to lose money, will the directors be liable for failure to exercise their duty of care and diligence? 

Yes, they will be liable for breaching this duty

The duty of care and diligence advocates for proper consideration, with regards to taking the 'reasonable person' perspective of what such a person would do given similar situations. In this view, decision made have to be measured,and have to be aimed at promoting the interests of the business. Now, if the directors have made bad decisions, the threshold of determining their culpability would be determined by also considering what another reasonable person,with the same level of experience,knowledge and skills would do. If they would have acted differently and taken necessary precautions, then the directors would be considered to have breached these duties, and would be liable