Transition metal carbene complexes are really important complexes in organometallic chemistry and they are used to produce important organic molecules like the ones in olefin metathesis for example. Two of these complexes are much more important and in fact they both have names: Fischer and Schrock type carbenes. You can read about these in any inorganic chemistry textbook. But, I think you can never find a better summary of these complexes. This paper, although a molecular modeling exercise, does a great job explaining the general properties of these two type carbenes. I wish textbooks were as clear and as simple as this article.