In $Fe_2(CO)_9$,the two iron atoms are

Zeise's salt is

Which of the following are organometallic compounds? $(I)$ Grignard reagent $(II)$ Trimethylboron $(III)$ Sodium methoxide $(IV)$ $TEL$

Which element does not form a mononuclear carbonyl?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: The metal-carbon bond in metal carbonyls possesses both $\sigma$ and $\pi$ character.
Reason $(R)$: The ligand-to-metal bond is a $\sigma$ bond and metal-to-ligand bond is a $\pi$ bond.
In the light of the above statements,choose the most appropriate answer from the options given below :

The Wilkinson catalyst,used for the hydrogenation of alkenes as a homogeneous catalyst,contains $......$.