The name of this compound is carbonyl(η5-cyclopentadienyl)(trimethylphosphane)cobalt. Note that, generally, the prefixes denoting the organic groups and any other ligands are placed in alphabetical order before the name of the metal.
The current version of Nomenclature of Inorganic Chemistry – IUPAC Recommendations 2005 (Red Book) presents the rules for formulating and naming coordination compounds (Chapter 9) and organometallic compounds (Chapter 10). An abridged version is included in the IUPAC Technical Report Brief guide to the nomenclature of inorganic chemistry. Pure Appl. Chem. 2015, 87(9–10), 1039–1049 as well as in the corresponding four-sided lift-out document, which is available as supplementary material.
In particular, the ‘η5-cyclopentadienyl’ term used here is explained in Section IR-10.2.5.1. as follows:
The special nature of the bonding of unsaturated hydrocarbons to metals via their π-electrons has led to the development of the ‘hapto’ nomenclature to designate unambiguously the unique bonding modes of the compounds so formed. (…) The Greek symbol η (eta) provides a topological description by indicating the connectivity between the ligand and the central atom. The number of contiguous atoms in the ligand coordinated to the metal is indicated by a right superscript numeral, e.g. η3 (‘eta three’ or ‘trihapto’), η4 (‘eta four’ or ‘tetrahapto’), η5 (‘eta five’ or ‘pentahapto’), etc. The symbol η is added as a prefix to the ligand name, or to that portion of the ligand name most appropriate to indicate the connectivity, (…)
The ligand name η5-cyclopentadienyl, although strictly speaking ambiguous, is acceptable as a short form of η5-cyclopenta-2,4-dien-1-yl, due to common usage.
These ligand names are enclosed in parentheses in the full name of a complex. Note the importance of making rigorous use of enclosing marks, etc. to distinguish the above bonding modes from the other four cases below. (…)