With the help of enterprise wsdl,  I have been trying to make upsert call in Contact where I need to relate the contact with Account . I have an external Id field on Account and trying to use it for upsert.  I am getting the below Error.

    <sf:exceptionCode>INVALID_TYPE</sf:exceptionCode>
               <sf:exceptionMessage>Must send a concrete entity type.</sf:exceptionMessage>
               <sf:row>-1</sf:row>
               <sf:column>-1</sf:column>

Below is the request, I am making....please let me know whats wrong here. I am making this request as sys admin.. and have all permission for this object. 

    <urn:upsert>
            <urn:externalIDFieldName>Account.Partner_Number_HDS__c</urn:externalIDFieldName>
                <urn:sObjects xsi:type="urn1:Contact"> <!--Zero or more repetitions:--> 
                 <FirstName>EnterA</FirstName>
                  <LastName>EnterB</LastName>
				<Department>Sales</Department>
				<Job_Title_HDS__c>Account Manager</Job_Title_HDS__c>
				<MobilePhone>456768985</MobilePhone>
				<Email>[email protected]</Email>
                <Account.Partner_Number_HDS__c>PA-74</Account.Partner_Number_HDS__c>
   </urn:sObjects>
</urn:upsert>

Thanks