The invention provides a cyclometallated complex comprising the structure of formula I wherein: M is a d-block transition metal; B is a five- or six-membered aryl or heteroaryl ring which is optionally substituted and optionally fused to one or more other aryl or heteroaryl rings; A is a five- or six-membered heteroaryl ring comprising at least three nitrogen atoms; R1 is a group other than hydrogen; n is zero or an integer equal to or greater than one; and A and B are optionally fused or linked by one or more covalent bonds. The invention also provides the use of such complexes in optoelectronic devices, and in particular in organic light emitting devices.